<init>()方法

<init>()方法，是虚拟机自己调用的，不在我们的程序中显性表示。

      那么<init>()的作用是什么？在什么时候被调用？

实际上，JVM为每一个类的每一个构造方法都创建一个<init>(）方法，用于初始化实例变量。

我们用一个例子来进行分析：

public class Test{
	private String name;
	private int age;
	
	public Test(){
	}
	
	public Test(String name){
		this.name=name;
	}	
	
	public Test(int age){
		this.age = age;
	}
	
	public int getAge(){
		return age;
	}
}

对这个类进行编译之后

利用指令：javap -verbose TestClass对其进行分析：

F:\java_cmd>javap -verbose Test
Classfile /F:/java_cmd/Test.class
  Last modified 2016-10-22; size 461 bytes
  MD5 checksum c2357af595833a04fc4d51146c91def8
  Compiled from "Test.java"
public class Test
  SourceFile: "Test.java"
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#20         //  java/lang/Object."<init>":(
   #2 = Fieldref           #4.#21         //  Test.name:Ljava/lang/String
   #3 = Fieldref           #4.#22         //  Test.age:I
   #4 = Class              #23            //  Test
   #5 = Class              #24            //  java/lang/Object
   #6 = Utf8               name
   #7 = Utf8               Ljava/lang/String;
   #8 = Utf8               age
   #9 = Utf8               I
  #10 = Utf8               <init>
  #11 = Utf8               ()V
  #12 = Utf8               Code
  #13 = Utf8               LineNumberTable
  #14 = Utf8               (Ljava/lang/String;)V
  #15 = Utf8               (I)V
  #16 = Utf8               getAge
  #17 = Utf8               ()I
  #18 = Utf8               SourceFile
  #19 = Utf8               Test.java
  #20 = NameAndType        #10:#11        //  "<init>":()V
  #21 = NameAndType        #6:#7          //  name:Ljava/lang/String;
  #22 = NameAndType        #8:#9          //  age:I
  #23 = Utf8               Test
  #24 = Utf8               java/lang/Object
{
  public Test();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object.
":()V
         4: return
      LineNumberTable:
        line 5: 0
        line 6: 4

  public Test(java.lang.String);
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object.
":()V
         4: aload_0
         5: aload_1
         6: putfield      #2                  // Field name:Ljava/lang/St
         9: return
      LineNumberTable:
        line 8: 0
        line 9: 4
        line 10: 9

  public Test(int);
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object.
":()V
         4: aload_0
         5: iload_1
         6: putfield      #3                  // Field age:I
         9: return
      LineNumberTable:
        line 12: 0
        line 13: 4
        line 14: 9

  public int getAge();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #3                  // Field age:I
         4: ireturn
      LineNumberTable:
        line 17: 0
}

我们可以从Class文件中发现，对于Test类中的每一个构造方法，JVM都有一条调用<init>()方法的指令存在