In java do you know what String does to '+'-JavaBase-php.cn

In java do you know what String does to '+'

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There is a simple code below

public class StringPlusTest{ public static void main(String[] args) { String s1 = "aaa"; String s2 = "bbb"; String s = "ccc" +s1 + s2 +"ddd"; }}

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Usejavap -c .\StringPlusTest.classto decompile and get

Compiled from "StringPlusTest.java"public class com.epoint.codetuning.test.StringPlusTest { public com.epoint.codetuning.test.StringPlusTest(); 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 aaa 2: astore_1 3: ldc #3 // String bbb 5: astore_2 6: new #4 // class java/lang/StringBuilder 9: dup 10: invokespecial #5 // Method java/lang/StringBuilder."":()V 13: ldc #6 // String ccc 15: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 18: aload_1 19: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 22: aload_2 23: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 26: ldc #8 // String ddd 28: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 31: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 34: astore_3 35: return}

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Among them,

ldc 把常量池中的项压入栈 astore_1 将引用类型或returnAddress类型值存入局部变量1 astore_2 将引用类型或returnAddress类型值存入局部变量2 new 创建一个新对象 dup 复制栈顶部一个字长内容 invokespecial 根据编译时类型来调用实例方法 aload_1 从局部变量1中装载引用类型值 aload_2 从局部变量2中装载引用类型值 astore_3 将引用类型或returnAddress类型值存入局部变量3

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For more instructions, please see the JVM instruction manual

For For Java, this code should be in principle:

public class StringPlusTest{ public static void main(String[] args) { String s1 = "aaa"; String s2 = "bbb"; String s = new StringBuilder().append("ccc").append(s1).append(s2).append("ddd").toString(); }}

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It can be seen that the implementation principle of using " " to splice strings in Java is to create a temporary StringBuilder object and call the append and toString methods.

Made some modifications to the above code

public class StringPlusTest{ public static void main(String[] args) { String s1 = "aaa"; String s2 = null; String s = "ccc" +s1 + s2 +"ddd"; System.out.println(s); }}

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What is the result?

You can get the following when running:

cccaaanullddd

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View the StringBuilder source code

@Overridepublic StringBuilder append(String str) { super.append(str); return this;}

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This method calls the method that inherits the parent class AbstractStringBuilder, and then goes to the parent class to view

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;}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;}

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## The #appendNull method simply has a capacity of 4 and appends a null string.

So the result is "cccaaanullddd".

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