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[add] add examples of generics · JavaDevelopRep/java-learning@d3e2afa · GitHub

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[add] add examples of generics
1 parent 3dafe09 commit d3e2afa

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‎java-base/src/main/java/com/brianway/learning/java/base/generics/ArrayMaker.java‎

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package com.brianway.learning.java.base.generics;
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import java.lang.reflect.Array;
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import java.util.Arrays;
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/**
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* 对于泛型中创建数组,使用Array.newInstance()是推荐的方式
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*/
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public class ArrayMaker<T> {
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private Class<T> kind;
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public ArrayMaker(Class<T> kind) {
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this.kind = kind;
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}
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@SuppressWarnings("unchecked")
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T[] create(int size) {
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return (T[]) Array.newInstance(kind, size);
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}
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public static void main(String[] args) {
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ArrayMaker<String> stringMaker =
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new ArrayMaker<String>(String.class);
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String[] stringArray = stringMaker.create(9);
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System.out.println(Arrays.toString(stringArray));
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}
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}
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/* Output:
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[null, null, null, null, null, null, null, null, null]
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*///:~

‎java-base/src/main/java/com/brianway/learning/java/base/generics/BasicBounds.java‎

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package com.brianway.learning.java.base.generics;
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public class BasicBounds {
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public static void main(String[] args) {
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Solid<Bounded> solid =
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new Solid<Bounded>(new Bounded());
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solid.color();
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solid.getY();
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solid.weight();
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}
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}
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interface HasColor {
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java.awt.Color getColor();
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}
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class Colored<T extends HasColor> {
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T item;
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Colored(T item) {
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this.item = item;
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}
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T getItem() {
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return item;
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}
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// The bound allows you to call a method:
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java.awt.Color color() {
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return item.getColor();
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}
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}
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class Dimension {
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public int x, y, z;
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}
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// This won't work -- class must be first, then interfaces:
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// class ColoredDimension<T extends HasColor & Dimension> {
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// Multiple bounds:
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class ColoredDimension<T extends Dimension & HasColor> {
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T item;
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ColoredDimension(T item) {
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this.item = item;
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}
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T getItem() {
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return item;
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}
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java.awt.Color color() {
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return item.getColor();
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}
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int getX() {
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return item.x;
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}
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int getY() {
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return item.y;
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}
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int getZ() {
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return item.z;
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}
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}
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interface Weight {
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int weight();
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}
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// As with inheritance, you can have only one
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// concrete class but multiple interfaces:
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class Solid<T extends Dimension & HasColor & Weight> {
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T item;
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Solid(T item) {
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this.item = item;
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}
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T getItem() {
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return item;
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}
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java.awt.Color color() {
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return item.getColor();
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}
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int getX() {
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return item.x;
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}
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int getY() {
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return item.y;
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}
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int getZ() {
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return item.z;
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}
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int weight() {
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return item.weight();
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}
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}
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class Bounded
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extends Dimension implements HasColor, Weight {
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public java.awt.Color getColor() {
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return null;
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}
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public int weight() {
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return 0;
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}
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}
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///:~

‎java-base/src/main/java/com/brianway/learning/java/base/generics/ClassTypeCapture.java‎

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package com.brianway.learning.java.base.generics;
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import java.util.HashMap;
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import java.util.Map;
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/**
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* 擦除的补偿
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* 显式传递类型的 Class 对象
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*/
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public class ClassTypeCapture<T> {
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//类型标签
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Class<T> kind;
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Map<String, Class<?>> types = new HashMap<>();
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public ClassTypeCapture(Class<T> kind) {
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this.kind = kind;
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}
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public boolean f(Object arg) {
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return kind.isInstance(arg);
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}
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public static void main(String[] args) {
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ClassTypeCapture<Building> ctt1 =
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new ClassTypeCapture<Building>(Building.class);
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System.out.println(ctt1.f(new Building()));
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System.out.println(ctt1.f(new House()));
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ClassTypeCapture<House> ctt2 =
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new ClassTypeCapture<House>(House.class);
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System.out.println(ctt2.f(new Building()));
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System.out.println(ctt2.f(new House()));
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}
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}
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class Building {
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}
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class House extends Building {
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}
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/* Output:
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true
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true
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false
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true
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*///:~

‎java-base/src/main/java/com/brianway/learning/java/base/generics/CovariantArrays.java‎

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package com.brianway.learning.java.base.generics;
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/**
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* 通配符相关类的基础类
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*/
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public class CovariantArrays {
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public static void main(String[] args) {
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Fruit[] fruit = new Apple[10];
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fruit[0] = new Apple(); // OK
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fruit[1] = new Jonathan(); // OK
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// Runtime type is Apple[], not Fruit[] or Orange[]:
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try {
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// Compiler allows you to add Fruit:
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fruit[0] = new Fruit(); // ArrayStoreException
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} catch (Exception e) {
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System.out.println(e);
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}
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try {
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// Compiler allows you to add Oranges:
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fruit[0] = new Orange(); // ArrayStoreException
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} catch (Exception e) {
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System.out.println(e);
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}
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}
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}
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class Fruit {
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}
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class Apple extends Fruit {
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}
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class Jonathan extends Apple {
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}
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class Orange extends Fruit {
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}
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/* Output:
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java.lang.ArrayStoreException: Fruit
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java.lang.ArrayStoreException: Orange
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*///:~

‎java-base/src/main/java/com/brianway/learning/java/base/generics/CreatorGeneric.java‎

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package com.brianway.learning.java.base.generics;
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/**
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* 末班方法设计模式
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*/
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public class CreatorGeneric {
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public static void main(String[] args) {
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Creator c = new Creator();
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c.f();
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}
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}
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abstract class GenericWithCreate<T> {
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final T element;
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GenericWithCreate() {
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element = create();
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}
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abstract T create();
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}
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class X {
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}
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class Creator extends GenericWithCreate<X> {
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X create() {
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return new X();
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}
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void f() {
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System.out.println(element.getClass().getSimpleName());
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}
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}
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/* Output:
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X
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*///:~

‎java-base/src/main/java/com/brianway/learning/java/base/generics/ErasedTypeEquivalence.java‎

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package com.brianway.learning.java.base.generics;
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import java.util.ArrayList;
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public class ErasedTypeEquivalence {
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public static void main(String[] args) {
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Class c1 = new ArrayList<String>().getClass();
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Class c2 = new ArrayList<Integer>().getClass();
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Class c3 = ArrayList.class;
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System.out.println(c1 == c2);
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System.out.println(c1 == c3);
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}
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}
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/* Output:
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true
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true
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*///:~

‎java-base/src/main/java/com/brianway/learning/java/base/generics/FactoryConstraint.java‎

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package com.brianway.learning.java.base.generics;//: generics/FactoryConstraint.java
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/**
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* 显式工厂
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*/
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public class FactoryConstraint {
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public static void main(String[] args) {
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new Foo2<Integer>(new IntegerFactory());
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new Foo2<Widget>(new Widget.Factory());
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}
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}
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interface FactoryI<T> {
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T create();
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}
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class Foo2<T> {
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private T x;
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public <F extends FactoryI<T>> Foo2(F factory) {
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x = factory.create();
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}
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}
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class IntegerFactory implements FactoryI<Integer> {
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public Integer create() {
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return 0;
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}
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}
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class Widget {
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public static class Factory implements FactoryI<Widget> {
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public Widget create() {
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return new Widget();
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}
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}
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}
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///:~

‎java-base/src/main/java/com/brianway/learning/java/base/generics/GenericArray.java‎

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package com.brianway.learning.java.base.generics;//: generics/GenericArray.java
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/**
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* 泛型数组
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*/
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public class GenericArray<T> {
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private T[] array;
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@SuppressWarnings("unchecked")
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public GenericArray(int sz) {
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array = (T[]) new Object[sz];
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}
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public void put(int index, T item) {
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array[index] = item;
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}
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public T get(int index) {
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return array[index];
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}
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// Method that exposes the underlying representation:
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public T[] rep() {
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return array;
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}
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public static void main(String[] args) {
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GenericArray<Integer> gai =
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new GenericArray<Integer>(10);
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// This causes a ClassCastException:
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//! Integer[] ia = gai.rep();
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// This is OK:
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Object[] oa = gai.rep();
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}
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} ///:~

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