两道小题目

两道小题目

@Test
public void test7() {
    String s1 = "abc";
    String s2 = "ab";
    String s3 = "c";
    String s4 = s2 + s3;

    String st0 = "helloworld";
    String st1 = "helloworld";
    String st2 = "hello" + "word";

    System.out.println(st0 == st2);
    System.out.println(s1 == (s2 + s3));
    System.out.println(s1 == s4);
}
​

@Test
public void test8() {
    int i = 0;
    i = i++;
    System.out.println(i);
}
​

• 第一题结果均为 false
• 第二题结果为 0

解析

利用 javap 反汇编查看 jvm 执行指令

第一题

public void test7();
  Code:
     0: ldc           #39                 // String abc
     2: astore_1
     3: ldc           #40                 // String ab
     5: astore_2
     6: ldc           #41                 // String c
     8: astore_3
     9: new           #19                 // class java/lang/StringBuilder
    12: dup
    13: invokespecial #20                 // Method java/lang/StringBuilder."<init>":()V
    16: aload_2
    17: invokevirtual #22                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    20: aload_3
    21: invokevirtual #22                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    24: invokevirtual #25                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
    27: astore        4
    29: ldc           #42                 // String helloworld
    31: astore        5
    33: ldc           #42                 // String helloworld
    35: astore        6
    37: ldc           #43                 // String helloword
    39: astore        7
    41: getstatic     #18                 // Field java/lang/System.out:Ljava/io/PrintStream;
    44: aload         5
    46: aload         7
    48: if_acmpne     55
    51: iconst_1
    52: goto          56
    55: iconst_0
    56: invokevirtual #33                 // Method java/io/PrintStream.println:(Z)V
    59: getstatic     #18                 // Field java/lang/System.out:Ljava/io/PrintStream;
    62: aload_1
    63: new           #19                 // class java/lang/StringBuilder
    66: dup
    67: invokespecial #20                 // Method java/lang/StringBuilder."<init>":()V
    70: aload_2
    71: invokevirtual #22                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    74: aload_3
    75: invokevirtual #22                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    78: invokevirtual #25                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
    81: if_acmpne     88
    84: iconst_1
    85: goto          89
    88: iconst_0
    89: invokevirtual #33                 // Method java/io/PrintStream.println:(Z)V
    92: getstatic     #18                 // Field java/lang/System.out:Ljava/io/PrintStream;
    95: aload_1
    96: aload         4
    98: if_acmpne     105
   101: iconst_1
   102: goto          106
   105: iconst_0
   106: invokevirtual #33                 // Method java/io/PrintStream.println:(Z)V
   109: return
​

可以看出字符串变量相加实际上 JVM 是优化为 java/lang/StringBuilder.append, 最终输出为 java/lang/StringBuilder.toString。

new String 则是直接引用堆中的对象，而 “” 则是引用常量池中的字符串, 这就是三个输出均为 false 的原因。

第二题

public void test8();
  Code:
     0: iconst_0  -- 将常量0加载进栈中, 入栈
     1: istore_1 -- 将栈顶int赋值给变量1，出栈
     2: iload_1 -- int变量1入栈
     3: iinc          1, 1 -- 变量1自增1,不会入栈 i = 1
     6: istore_1 -- 将栈顶int赋值给变量1，出栈 i = 0
     7: getstatic     #18                 // Field java/lang/System.out:Ljava/io/PrintStream;
    10: iload_1 -- int变量1入栈
    11: invokevirtual #29                 // Method java/io/PrintStream.println:(I)V
    14: return
​

由于执行完 iinc 1, 1 指令之后并没有将变量 i 入栈，然后直接将栈顶的 0 赋值给了变量 i

而栈顶的 0 来自于 i++ 的 0。这就导致了最终的输出结果0。

对比

@Test
public void test8() {
    int i = 0;
    i++;
    System.out.println(i);
}
​

public void test8();
  Code:
     0: iconst_0
     1: istore_1
     2: iinc          1, 1
     5: getstatic     #18                 // Field java/lang/System.out:Ljava/io/PrintStream;
     8: iload_1
     9: invokevirtual #29                 // Method java/io/PrintStream.println:(I)V
    12: return
​