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关于字符串和字符串常量相加

关于字符串和字符串常量相加

作者: JurlyL | 来源:发表于2019-01-17 11:34 被阅读0次

    先贴一段代码

    public static void main(String[] args){
    String s1 = "abc";
    String s2 = "a";
    String s3 = "bc";
    String s4 = "a" + "bc";
    String s5 = "a" + s3;
    String s6 = s2 + s3;
    System.out.println(s1 == s4); //true
    System.out.println(s1 == s5); //false
    System.out.println(s1 == s6); //false
    }

    程序输出最后是true,false,false。之前一直不太懂为什么是这个输出结果,这几天在学字节码,于是查看了这段程序的字节码。

    public class com.neo.test.Test {
    public com.neo.test.Test();
    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 abc
    2: astore_1
    3: ldc #3 // String a
    5: astore_2
    6: ldc #4 // String bc
    8: astore_3
    9: ldc #2 // String abc
    11: astore 4
    13: new #5 // class java/lang/StringBuilder
    16: dup
    17: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
    20: ldc #3 // String a
    22: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    25: aload_3
    26: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    29: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
    32: astore 5
    34: new #5 // class java/lang/StringBuilder
    37: dup
    38: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
    41: aload_2
    42: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    45: aload_3
    46: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    49: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
    52: astore 6
    54: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
    57: aload_1
    58: aload 4
    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 5
    77: if_acmpne 84
    80: iconst_1
    81: goto 85
    84: iconst_0
    85: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
    88: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
    91: aload_1
    92: aload 6
    94: if_acmpne 101
    97: iconst_1
    98: goto 102
    101: iconst_0
    102: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
    105: return
    }

    从字节码中可以看出,字符串"+"其实是调用的StringBuilder的append方法,在每次执行"+"的时候都会先new一个StringBuilder对象来存储需要相加的字符串,最后通过toString()方法来转换成String。因此得到的String是一个新的对象。所以s1和s5,s6的地址是不同的。

    在代码中s4 = "a" +"bc"对应的字节码是9: ldc #2 // String abc
    ,和第一句s1 = "abc"对应的字节码是一样的,在网上查找之后,在字符串常量相加时候,预编译时候会把"+"优化,直接把两个字符串常量自动合成一个字符串常量,因此s1和s4其实是常量池中同一个字符串"abc"的引用,因此返回结果是true。

    这仅仅是学习记录。。。可能有表达出错或者我理解出错的地方,如果有大佬看到了本咸鱼的笔记emmmm欢迎指正~

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