Viewing: www.runoob.com

1Class

public class Car { ... }

2Object

Car myCar = new Car();

3Inheritance

public class Dog extends Animal { ... }

4Encapsulation

private String name; 
public String getName() { return name; }

5Polymorphism

- public int add(int a, int b) { ... } public double add(double a, double b) { ... }
- @Override public void makeSound() { System.out.println("Meow"); }

6Abstraction

- public abstract class Shape { abstract void draw(); }
- public interface Animal { void eat(); }

7Interface

public interface Drivable { void drive(); }

8Method

public void displayInfo() { System.out.println("Info"); }

9Method Overloading

public class MathUtils {
    public int add(int a, int b) {
        return a + b;
    }

    public double add(double a, double b) {
        return a + b;
    }
}

boygirlclassobject

[object-class.jpg]

classobject

[class-object2020-10-27.png]

Java

[20210105-java-object-1.png]

Java

public class Dog { String breed; int size; String colour; int age; void eat() { } void run() { } void sleep(){ } void name(){ } }

static

eat()run()sleep() name() Dog

Java

public class Puppy{ public Puppy(){ } public Puppy(String name){ // name } }

Java new

public class Puppy{ public Puppy(String name){ //name System.out.println(" : " + name ); } public static void main(String[] args){ // Puppy Puppy myPuppy = new Puppy( "tommy" ); } }

 : tommy

/* */ Object referenceVariable = new Constructor(); /* */ referenceVariable.variableName; /* */ referenceVariable.methodName();

Object Object

Puppy.java

public class Puppy { private int age; private String name; // public Puppy(String name) { this.name = name; System.out.println(" : " + name); } // age public void setAge(int age) { this.age = age; } // age public int getAge() { return age; } // name public String getName() { return name; } // public static void main(String[] args) { // Puppy myPuppy = new Puppy("Tommy"); // age myPuppy.setAge(2); // age int age = myPuppy.getAge(); System.out.println(" : " + age); // getter System.out.println(" : " + myPuppy.getAge()); } }

 : tommy
 : 2
 : 2

import package

public
public
public public EmployeeEmployee.java
package
import package package import
import package

final

Java

import

Java Java import

java_installation/java/io

import java.io.*;

Employee EmployeeTest

Employee.java

Employee nameagedesignation salary

Employee.java

import java.io.*; public class Employee { private String name; private int age; private String designation; private double salary; // Employee public Employee(String name) { this.name = name; } // age public void setAge(int age) { this.age = age; } // age public int getAge() { return age; } // designation public void setDesignation(String designation) { this.designation = designation; } // designation public String getDesignation() { return designation; } // salary public void setSalary(double salary) { this.salary = salary; } // salary public double getSalary() { return salary; } // public void printEmployee() { System.out.println(this); } // toString @Override public String toString() { return ": " + name + "\n" + ": " + age + "\n" + ": " + designation + "\n" + ": " + salary; } }

Java main main

EmployeeTest 2 Employee

EmployeeTest.java

import java.io.*; public class EmployeeTest { public static void main(String[] args) { // Employee empOne = new Employee("RUNOOB1"); Employee empTwo = new Employee("RUNOOB2"); // empOne.setAge(26); empOne.setDesignation(""); empOne.setSalary(1000); empOne.printEmployee(); empTwo.setAge(21); empTwo.setDesignation(""); empTwo.setSalary(500); empTwo.printEmployee(); } }

EmployeeTest

$ javac EmployeeTest.java Employee.java
$ java EmployeeTest 
:RUNOOB1
:26
:
:1000.0
:RUNOOB2
:21
:
:500.0

AI ...

Java

15

#0
   stinkaroo
  190***[email protected]
713
javapublic
stinkaroo
   stinkaroo
  190***[email protected]
9 (2017-05-08)
#0
   LadyLeane
  q-b***sn.com
416
package c++ namespace Java package class package aaa.bbb.ccc .java ./aaa/bbb/ccc/

import package ./aaa/bbb/ccc/ A importAnew aaa.bbb.ccc.A() import aaa.bbb.ccc.A new A() aaa.bbb.ccc.
LadyLeane
   LadyLeane
  q-b***sn.com
9 (2017-08-02)
#0

  253***[email protected]

279
JAVAPublic?

java public main (main) C main() public public
() public public public public() public

  253***[email protected]

9 (2017-09-15)
#0
   pxn626
  790***[email protected]

691
static

   1


   2

   3


   4

static ()

   1
   2.
    3

   1
   2

   3thissuper
      thisthis

   1



   2




   1

   2

   3

   4


pxn626
   pxn626
  790***[email protected]

9 (2017-09-28)

251

public class  ClassName{
    public void printNumber{
        int a;
    }
    // 
}

public class  ClassName{
    int a;
    public void printNumber{
        // 
    }
}

3. static

public class  ClassName{
    static int a;
    public void printNumber{
        // 
    }
}

Demetris9 (2017-11-14)

#0
   ycxchkj
  xch***163.com
131
1
2
3new
4JavaJava
new
1
2
3
ycxchkj
   ycxchkj
  xch***163.com
9 (2018-01-27)
#0
   Lynn
  276***[email protected]

55

Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object.

Oracle
Lynn
   Lynn
  276***[email protected]

9 (2018-03-18)

java

//
Public class People{
    int age = 23;
    Public void getAge(){
        System.out.print("the age is "+age);
    }
}

//
People xiaoMing = new People(); // People() People
xiaoMing.getAge();//

//
Public class People{
    int age = 23;
        Public void getAge(){
        System.out.print("the age is "+ age);
    }
    // 
    Public People(int a){
        this.age = a; 
    }
}

//
People xiaoMing = new People(20); // 
xiaoMing.getAge(); // 20

lllunaticer9 (2018-03-31)

#0
   2333
  135***[email protected]
85

1.

2.

3., this.color

this

new Student(); new

2333
   2333
  135***[email protected]
9 (2018-04-13)

102

public class Person {
    private int blood;
    private Heart heart;
}

public class Heart {
    private int blood;
    public void test() {
        System.out.println(blood);
    }
}

public class Person {
    private int blood;
    public class Heart {
        public void test() {
            System.out.println(blood);
        }
    }

    public class Brain {
        public void test() {
            System.out.println(blood);
        }
    }
}

//
class Out {
    private int age = 12;
     
    //
    class In {
        public void print() {
            System.out.println(age);
        }
    }
}
 
public class Demo {
    public static void main(String[] args) {
        Out.In in = new Out().new In();
        in.print();
        //
        /*
        Out out = new Out();
        Out.In in = out.new In();
        in.print();
        */
    }
}

.class Out.class Out$In.class

$ Out.In

Out.In in = new Out().new In()

1. Out
2.

2

java

Java

tfbyly8 (2018-04-18)

Java this super

this this
super

class Person { 
    public static void prt(String s) { 
       System.out.println(s); 
    } 
   
    Person() { 
       prt("· "+"A Person."); 
    }//(1) 
    
    Person(String name) { 
       prt("· "+"A person's name is " + name); 
    }//(2) 
} 
    
public class Chinese extends Person { 
    Chinese() { 
       super(); // 1 
       prt("·”“ "+"A chinese coder."); 
    } 
    
    Chinese(String name) { 
       super(name);// 2 
       prt("·”“ "+"his name is " + name); 
    } 
    
    Chinese(String name, int age) { 
       this(name);// 3 
       prt("his age is " + age); 
    } 
    
    public static void main(String[] args) { 
       Chinese cn = new Chinese(); 
       cn = new Chinese("codersai"); 
       cn = new Chinese("codersai", 18); 
    } 
}

· A Person.
·”“ A chinese coder.
· A person's name is codersai
·”“ his name is codersai
· A person's name is codersai
·”“ his name is codersai
his age is 18

8 (2018-07-06)

JAVA C++

//
Public class People{
    int age = 23;
        Public void getAge(){
        System.out.print("the age is "+ age);
    }
    // 
    Public People(int a){
        this.age = a; 
    }
}

//
People xiaoMing = new People(20); // 
People xiaoMing2 = new People();  // ERROR
xiaoMing.getAge(); // 20

samcyang8 (2018-09-11)

#0
   Phanio
  pen***[email protected]

36
CMD Java GBK

javac -encoding UTF-8 .java

JDK -encoding JAVA java.exe JAVA JDK file.encoding win2k GBK JDK JAVA file.encoding JAVA UNICODE javac UNICODE class .class UNICODE JDK UNICODE .class .class : javac -encoding gbk xx.java
Phanio
   Phanio
  pen***[email protected]

8 (2018-12-25)

117

class A{}  //

static

eat()run() sleep() Dog

new :

A obj = new A();
A obj1 = A(); 
A obj2 = new A();

public (){}  
public (String name){}

/*  */
Object referenceVariable = new Constructor();
/*  */
referenceVariable.variableName;
/*  */
referenceVariable.methodName();

public class  ClassName{
    public void printNumber{
        int a;
    }
    // 
}

public class  ClassName{
    int a;
    public void printNumber{
        // 
    }
}

3. static

public class  ClassName{
    static int a;
    public void printNumber{
        // 
    }
}

Ekko4047 (2019-09-26)

426

LOL

lol

public class Hero {
    String name; //
    int attackDamage; //
    int abilityPower; //
    int armor; //
    int magicResistance; //
    float attackSpeed; //
    int cooldownReduction; //
    int criticalStrike; //
    int moveSpeed; //
    int hp; //
    int mp; //
}

public class Hero {
    public void DestroyTower(){
        System.out.println("");
    }
    public void Keng(){
        System.out.println("");
    }
    public void Kb(){
        System.out.println("");
    }
    public void Dance(){
        System.out.println("");
    }
}

(garen) main new

public static void main(String[] args) {
    Hero garen = new Hero();
    garen.name = "";
    garen.attackDamage = 71;
    garen.abilityPower = 0;
    garen.armor = 36;
    garen.magicResistance = 32;
    garen.attackSpeed = 0.69f;
    garen.cooldownReduction = 0;
    garen.criticalStrike = 0;
    garen.moveSpeed = 350;
    garen.hp = 600;
    garen.mp = 0;
}

jidaojiuyou7 (2019-10-15)

--

Java

Java

Java

Java

Java

Puppy.java

Java

import

Employee.java

EmployeeTest.java

15

2