1.6.1 拼接结果的存储和比较问题

• 原则：

  • 常量+常量：结果是常量池。
  • 常量和变量 或 变量和变量：结果是堆。
  • 拼接后调用 intern 方法：结果在常量池。

• 示例：

package com.github.string.demo7;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "world";
        String s3 = "helloworld";
        String s4 = (s1 + "world").intern(); // 将拼接的结果放在常量池中
        String s5 = (s1 + s2).intern();
        System.out.println(s3 == s4); // true
        System.out.println(s3 == s5); // true
    }
}

• 示例：

package com.github.string.demo8;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        final String s1 = "hello";
        final String s2 = "world";
        String s3 = "helloworld";
        String s4 = s1 + "world"; // s1是常量，"world"也是常量，所以s4是常量
        String s5 = s1 + s2; // s1是常量，s2是常量，所以s5是常量
        String s6 = "hello" + "world"; // "hello"是常量，"world"是常量，所以s6是常量
        System.out.println(s3 == s4); // true
        System.out.println(s3 == s5); // true
        System.out.println(s3 == s6); // true
    }
}

• 示例：

package com.github.string.demo9;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "world";
        String s3 = "helloworld";
        String s4 = s1 + "world"; // s1是变量，"world"是常量，所以s4是变量
        String s5 = s1 + s2; // s1是变量，s2是变量，所以s5是变量
        String s6 = "hello" + "world"; // "hello"是常量，"world"是常量，所以s6是常量
        System.out.println(s3 == s4); // false
        System.out.println(s3 == s5); // false
        System.out.println(s3 == s6); // true
    }
}

1.6.2 拼接效率问题

• 示例：

package com.github.string.demo10;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        String str = "0";
        for (int i = 0; i < 5; i++) {
            str += i;
        }
        System.out.println("str = " + str);
    }
}

不过，现在的 JDK 版本，都会使用可变字符序列对如上的代码优化，不信，反编译查看字节码：

javap -c Test.class

Compiled from "Test.java"
public class com.github.string.demo10.Test {
  public com.github.string.demo10.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 0
       2: astore_1
       3: iconst_0
       4: istore_2
       5: iload_2
       6: iconst_5
       7: if_icmpge     35
      10: new           #3                  // class java/lang/StringBuilder
      13: dup
      14: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
      17: aload_1
      18: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      21: iload_2
      22: invokevirtual #6                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      25: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      28: astore_1
      29: iinc          2, 1
      32: goto          5
      35: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
      38: new           #3                  // class java/lang/StringBuilder
      41: dup
      42: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
      45: ldc           #9                  // String str =
      47: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      50: aload_1
      51: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      54: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      57: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      60: return
}

1.6.3 concat 方法拼接

• concat() 方法拼接，哪怕是两个常量对象拼接，结果也是在堆中。

• 示例：

package com.github.string.demo19;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "world";
        String s3 = "helloworld";
        String s4 = "hello".concat("world");
        String s5 = "hello" + "world";
        System.out.println(s3 == s4); // false
        System.out.println(s3 == s5); // true
        System.out.println(s1 + s2 == s5); // false
    }
}