String常见问题

• String的不变性
  英文版：https://www.programcreek.com/2009/02/diagram-to-show-java-strings-immutability/
  中文版：https://www.hollischuang.com/archives/1230

• JDK6和JDK7中的substring的原理及区别
  英文版：https://www.programcreek.com/2013/09/the-substring-method-in-jdk-6-and-jdk-7/
  中文版：https://www.hollischuang.com/archives/1232

• String对“+”的重载
  为什么不建议使用+连接字符串呢，看如下代码：

public void testString() {
       String a = "temp";
       for (int i = 0; i < 10; i++) {
           a += i;
       }
   }

javap -c Test.class

 public void testString();
   Code:
      0: ldc           #2                  // String temp
      2: astore_1
      3: iconst_0
      4: istore_2
      5: iload_2
      6: bipush        10
      8: if_icmpge     36
     11: new           #3                  // class java/lang/StringBuilder
     14: dup
     15: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
     18: aload_1
     19: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
     22: iload_2
     23: invokevirtual #6                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
     26: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
     29: astore_1
     30: iinc          2, 1
     33: goto          5
     36: return

可以从字节码中看出满足循环条件，从5处开始每次使用+，就会new一个StringBuilder对象，然后使用StringBuilder.append()，最后再用toString()转换，这样就会创建很多没有必要的StringBuilder对象，增加系统开销。

• Java 7中switch支持String

public void testSwitch() {
      String test = "test";
        switch (test) {
            case "t":
                System.out.println("t");
                break;
            default:
                System.out.println("default");
        }
    }

查看字节码

    Code:
       0: ldc           #2                  // String test
       2: astore_1
       3: aload_1
       4: astore_2
       5: iconst_m1
       6: istore_3
       7: aload_2
       8: invokevirtual #3                  // Method java/lang/String.hashCode:()I
      11: lookupswitch  { // 1
                   116: 28
               default: 39
          }
      28: aload_2
      29: ldc           #4                  // String t
      31: invokevirtual #5                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
      34: ifeq          39
      37: iconst_0
      38: istore_3
      39: iload_3
      40: lookupswitch  { // 1
                     0: 60
               default: 71
          }
      60: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      63: ldc           #4                  // String t
      65: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      68: goto          79
      71: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      74: ldc           #8                  // String default
      76: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      79: return

从8处可以看出，switch(test)执行的是String.hashCode()，即switch(hashCode值)，可以查看String的源码，hashCode()返回的是int类型，switch其实就是对整型支持。case "t" 即是31处"t".equals(test)

延伸-switch对枚举的支持

public class TestEnum {
    public static void main(String[] args) {
        Type type = Type.NUMBER;
        switch (type) {
            case NUMBER:
                System.out.println("number");
            case STRING:
                System.out.println("string");
        }
    }
}

enum Type {
    NUMBER,
    STRING
}

查看字节码

Compiled from "TestEnum.java"
public class demo.TestEnum {
  public demo.TestEnum();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field demo/Type.NUMBER:Ldemo/Type;
       3: astore_1
       4: getstatic     #3                  // Field demo/TestEnum$1.$SwitchMap$demo$Type:[I
       7: aload_1
       8: invokevirtual #4                  // Method demo/Type.ordinal:()I
      11: iaload
      12: lookupswitch  { // 2
                     1: 40
                     2: 48
               default: 56
          }
      40: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
      43: ldc           #6                  // String number
      45: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      48: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
      51: ldc           #8                  // String string
      53: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      56: return
}

从12处可以看出，枚举值对应成顺序整数，也再次印证我们之前说的，switch其实就是对整型的支持。

Compiled from "TestEnum.java"
class demo.TestEnum$1 {
  static final int[] $SwitchMap$demo$Type;

  static {};
    Code:
       0: invokestatic  #1                  // Method demo/Type.values:()[Ldemo/Type;
       3: arraylength
       4: newarray       int
       6: putstatic     #2                  // Field $SwitchMap$demo$Type:[I
       9: getstatic     #2                  // Field $SwitchMap$demo$Type:[I
      12: getstatic     #3                  // Field demo/Type.NUMBER:Ldemo/Type;
      15: invokevirtual #4                  // Method demo/Type.ordinal:()I
      18: iconst_1
      19: iastore
      20: goto          24
      23: astore_0
      24: getstatic     #2                  // Field $SwitchMap$demo$Type:[I
      27: getstatic     #6                  // Field demo/Type.STRING:Ldemo/Type;
      30: invokevirtual #4                  // Method demo/Type.ordinal:()I
      33: iconst_2
      34: iastore
      35: goto          39
      38: astore_0
      39: return
    Exception table:
       from    to  target type
           9    20    23   Class java/lang/NoSuchFieldError
          24    35    38   Class java/lang/NoSuchFieldError
}

ps:字节码真是个好东西，原来好多不懂的原理，都可以通过字节码看出个一二三来。