Java中字符串String加法解析

测试代码：

public static void main(String[] args) {
        String a = "hello2";
        final String b = "hello";
        String d = "hello";
        String c = b + 2;
        // 由于b是常量所以编译期，编译器会直接将b替换为其值，并直接拼接成"Hello2"赋值给c，这又叫做“常量优化”
        // 故在编译期就能确定b值
        String e = d + 2;
        // 由于赋值语句右侧使用的不同类型常量赋值，因此会使用StringBuilder实现字符串拼接
        // 会先调用其append方法最后调用toString方法，而toString方法是new String并返回，即在堆中创建对象
        // 故e的值需要运行时确定
        String f = "hello" + "2";
        // 而f的赋值语句右侧也都是字面量，或者说常量，同字符串c的解释，故在编译期就能确定f值
        // 故最终的输出结果为：
        System.out.println(a == c);// true
        System.out.println(a == e);// false
        System.out.println(a == f);// true
    }

反汇编class文件输出：

C:\Users\DELL\Desktop>javap -c Solution.class
Compiled from "Solution.java"
public class LeetCodeStudy.Solution {
  public LeetCodeStudy.Solution();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String hello2
       2: astore_1
       3: ldc           #3                  // String hello
       5: astore_2
       6: ldc           #3                  // String hello
       8: astore_3
       9: ldc           #2                  // String hello2
      11: astore        4
      13: new           #4                  // class java/lang/StringBuilder
      16: dup
      17: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      20: aload_3
      21: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: iconst_2
      25: invokevirtual #7                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      28: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      31: astore        5
      33: ldc           #2                  // String hello2
      35: astore        6
      37: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
      40: aload_1
      41: aload         4
      43: if_acmpne     50
      46: iconst_1
      47: goto          51
      50: iconst_0
      51: invokevirtual #10                 // Method java/io/PrintStream.println:(Z)V
      54: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
      57: aload_1
      58: aload         5
      60: if_acmpne     67
      63: iconst_1
      64: goto          68
      67: iconst_0
      68: invokevirtual #10                 // Method java/io/PrintStream.println:(Z)V
      71: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
      74: aload_1
      75: aload         6
      77: if_acmpne     84
      80: iconst_1
      81: goto          85
      84: iconst_0
      85: invokevirtual #10                 // Method java/io/PrintStream.println:(Z)V
      88: return
}

参考文档：

String 相加解析原理
String字符串相加的问题
java 字符串拼接为什么要用 StringBuilder 而不直接用 String 相加连接