JavapTest.java

public class JavapTest {
	public static void main(String[] args) throws Exception {		
		int i = 0;
		System.out.println(i++);
		System.out.println(++i);
		String str = "a"+"b";
		System.out.println(str);
		str = "a"+i+"b";	
		System.out.println(str);
		StringBuilder sb = new StringBuilder();
		sb.append("a").append(i).append("b");	
		System.out.println(sb);
	}
}


编译
javac JavapTest.java
运行
java JavapTest
查看字节码
javap -c JavapTest

String str = "a"+"b";
转换成字符串常量 ab


str = "a"+i+"b";
字符串拼接转换成 StringBuilder 



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

  public static void main(java.lang.String[]) throws java.lang.Exception;
    Code:
       0: iconst_0
       1: istore_1
       2: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       5: iload_1
       6: iinc          1, 1
       9: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      12: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      15: iinc          1, 1
      18: iload_1
      19: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      22: ldc           #4                  // String ab
      24: astore_2
      25: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      28: aload_2
      29: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      32: new           #6                  // class java/lang/StringBuilder
      35: dup
      36: invokespecial #7                  // Method java/lang/StringBuilder."":()V
      39: ldc           #8                  // String a
      41: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      44: iload_1
      45: invokevirtual #10                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      48: ldc           #11                 // String b
      50: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      53: invokevirtual #12                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      56: astore_2
      57: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      60: aload_2
      61: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      64: new           #6                  // class java/lang/StringBuilder
      67: dup
      68: invokespecial #7                  // Method java/lang/StringBuilder."":()V
      71: astore_3
      72: aload_3
      73: ldc           #8                  // String a
      75: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      78: iload_1
      79: invokevirtual #10                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      82: ldc           #11                 // String b
      84: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      87: pop
      88: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      91: aload_3
      92: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      95: return
}