---
title: Java
shortTitle: Java
category:
- Java
tag:
- &
description: JavaJavaJava
head:
- - meta
- name: keywords
content: Java,Java SE,Java,Java,Java,Java,Java,,java,String,,java,java string
---
# 4.9
[Java ](https://tobebetterjavaer.com/pdf/ali-java-shouce.html) StringBuilder `append()` +
[](https://tobebetterjavaer.com/string/builder-buffer.html)
PS
### javap +
+ Java + Intellij IDEA
```java
class Demo {
public static void main(String[] args) {
String chenmo = "";
String wanger = "";
System.out.println(chenmo + wanger);
}
}
```
Java 8 `javap -c Demo.class`
```
Compiled from "Demo.java"
class Demo {
Demo();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #2 // String
2: astore_1
3: ldc #3 // String
5: astore_2
6: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder."":()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
}
```
new class `java/lang/StringBuilder` 9 StringBuilder
17 invokevirtual StringBuilder `append()`
chenmo"" StringBuilder
21 `append()` wanger"" StringBuilder
Java
```java
class Demo {
public static void main(String[] args) {
String chenmo = "";
String wanger = "";
System.out.println((new StringBuilder(chenmo)).append(wanger).toString());
}
}
```
+ StringBuilder `append()`
Java 9 Java 11
Java 11
```
Compiled from "Demo.java"
public class com.itwanger.thirtyseven.Demo {
public com.itwanger.thirtyseven.Demo();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #2 // String
2: astore_1
3: iconst_0
4: istore_2
5: iload_2
6: bipush 10
8: if_icmpge 41
11: new #3 // class java/lang/String
14: dup
15: ldc #4 // String
17: invokespecial #5 // Method java/lang/String."":(Ljava/lang/String;)V
20: astore_3
21: ldc #6 // String
23: astore 4
25: aload_1
26: aload_3
27: aload 4
29: invokedynamic #7, 0 // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
34: astore_1
35: iinc 2, 1
38: goto 5
41: return
}
```
29 `invokedynamic`Bootstrap Method BSMBSM CallSite `invokedynamic` `invokedynamic` CallSite CallSite MethodHandle holder `StringConcatFactory.makeConcatWithConstants()`
Java 9 JDK + Java 8
### StringBuilder.append
Java StringBuilder `append()` + + StringBuilder Java
StringBuilder `append()`
```java
class Demo {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
for (int i = 1; i < 10; i++) {
String chenmo = "";
String wanger = "";
sb.append(chenmo);
sb.append(wanger);
}
System.out.println(sb);
}
}
```
for +
```java
String result = "";
for (int i = 0; i < 100000; i++) {
result += "";
}
```
for StringBuilder `append()`
```java
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 100000; i++) {
sb.append("");
}
```
6212 1
### append
StringBuilder `append()`
```java
public StringBuilder append(String str) {
super.append(str);
return this;
}
```
3 AbstractStringBuilder `append()`
```java
public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
}
```
1 nullnull`appendNull()`
```java
private AbstractStringBuilder appendNull() {
int c = count;
ensureCapacityInternal(c + 4);
final char[] value = this.value;
value[c++] = 'n';
value[c++] = 'u';
value[c++] = 'l';
value[c++] = 'l';
count = c;
return this;
}
```
2
3`ensureCapacityInternal()`
```java
private void ensureCapacityInternal(int minimumCapacity) {
// overflow-conscious code
if (minimumCapacity - value.length > 0) {
value = Arrays.copyOf(value,
newCapacity(minimumCapacity));
}
}
```
4 str value
```java
str.getChars(0, len, value, count)
```
5 count
### String.concat
+ StringBuilder `append()`
String `concat()` StringBuilder `append()`
```java
String chenmo = "";
String wanger = "";
System.out.println(chenmo.concat(wanger));
```
`concat()`
```java
public String concat(String str) {
int otherLen = str.length();
if (otherLen == 0) {
return this;
}
int len = value.length;
char buf[] = Arrays.copyOf(value, len + otherLen);
str.getChars(buf, len);
return new String(buf, true);
}
```
1 0
2 value buf
3 str buf
`+` `concat()` null NullPointerException+ null null
"" concat , `new StringBuilder`
`concat()`
### String.join
String `join()`
```java
String chenmo = "";
String wanger = "";
String cmower = String.join("", chenmo, wanger);
System.out.println(cmower);
```
```java
String message = String.join("-", "", "", "");
```
`--`
join
```java
public static String join(CharSequence delimiter, CharSequence... elements) {
Objects.requireNonNull(delimiter);
Objects.requireNonNull(elements);
// Number of elements not likely worth Arrays.stream overhead.
StringJoiner joiner = new StringJoiner(delimiter);
for (CharSequence cs: elements) {
joiner.add(cs);
}
return joiner.toString();
}
```
StringJoiner for-each `toString()` String
### StringUtils.join
`org.apache.commons.lang3.StringUtils` `join()`
```java
String chenmo = "";
String wanger = "";
StringUtils.join(chenmo, wanger);
```
NullPointerException
```java
StringUtils.join(null) = null
StringUtils.join([]) = ""
StringUtils.join([null]) = ""
StringUtils.join(["a", "b", "c"]) = "abc"
StringUtils.join([null, "", "a"]) = "a"
```
```java
public static String join(final Object[] array, String separator, final int startIndex, final int endIndex) {
if (array == null) {
return null;
}
if (separator == null) {
separator = EMPTY;
}
final StringBuilder buf = new StringBuilder(noOfItems * 16);
for (int i = startIndex; i < endIndex; i++) {
if (i > startIndex) {
buf.append(separator);
}
if (array[i] != null) {
buf.append(array[i]);
}
}
return buf.toString();
}
```
StringBuilder
Java 9 + [](https://tobebetterjavaer.com/jvm/zijiema-zhiling.html)
---
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**** **111**
