Anonymous subclass

 A simple example:

public class Test {

    public static void main(String[] args) throws Exception {
        List<String> list = new ArrayList<String>() {
            {
                add("a");
                add("b");
            }
        };
    }
}

We then compiled after the class file out to look, what happened. First of all see is a Test.java compiled a 2 class

isaac-mbp:basic foxty$ ls -l
total 16
-rw-r--r--  1 foxty  staff  542  2  9 22:08 Test$1.class
-rw-r--r--  1 foxty  staff  629  2  9 22:08 Test.class

Then we look at what Test.class itself is compiled into the direct use jdk javap comes on the line.

isaac-mbp:basic foxty$ javap -c Test.class 
Compiled from "Test.java"
public class com.foxty.basic.Test {
  public com.foxty.basic.Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]) throws java.lang.Exception;
    Code:
       0: new           #2                  // class com/foxty/basic/Test$1-这里创建了一个Test$1的类
       3: dup
       4: invokespecial #3                  // Method com/foxty/basic/Test$1."<init>":()V
       7: astore_1
       8: return
}

Then look at what the other Test $ 1.class, from the anti-compiled code below will know this Test $ 1 is a subclass of ArrayList. Combined with the original java code, I know this is actually written to create an anonymous subclass of ArrayList of it. Inside the braces is equivalent to that layer you add to the class initialization of a code block, this block of code will be automatically incorporated into the class init method, which is the object's constructor.

isaac-mbp:basic foxty$ javap -c Test\$1.class 
Compiled from "Test.java"
final class com.foxty.basic.Test$1 extends java.util.ArrayList<java.lang.String> {
  com.foxty.basic.Test$1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/util/ArrayList."<init>":()V
       4: aload_0
       5: ldc           #2                  // String a
       7: invokevirtual #3                  // Method add:(Ljava/lang/Object;)Z
      10: pop
      11: aload_0
      12: ldc           #4                  // String b
      14: invokevirtual #3                  // Method add:(Ljava/lang/Object;)Z
      17: pop
      18: return
}