public class Test {
  public static void main(String[] args) {
    final String str1 = "HelloWorld";
    final String str2 = "Hello" + "World";
    System.out.println(str1 == str2);//true
    
    final String str3 = "Hello" + String.valueOf("World");
    System.out.println(str1 == str3);//false
  }
}

public class Test {
  
  final String str1 = "HelloWorld";
  final String str2 = "Hello" + "World";
  final String str3;
  final String str4;
  {
    str3 = "HelloWorld";
  }
  {
    System.out.println(str1 == str2);//true
    System.out.println(str1 == str3);//true
//    System.out.println(str1 == str4);//compile error
  }
  public Test() {
    str4 = "HelloWorld";
    System.out.println(str1 == str4);//true
  }
  
  public static void main(String[] args) {
    new Test();
  }
}

public class Test {
   final String str1;//compile error---没有显示的使用①②③中的方式进行初始化
   String str2;
 }

public class Test {
  final String str1 = "Hello";//定义实例变量时指定初始值
  
  final String str2;//非静态初始化块中对实例变量进行初始化
  final String str3;//构造器中对实例变量进行初始化
  
  {
    str2 = "Hello";
  }
  public Test() {
    str3 = "Hello";
  }
  
  public void show(){
    System.out.println(str1 + str1 == "HelloHello");//true
    System.out.println(str2 + str2 == "HelloHello");//false
    System.out.println(str3 + str3 == "HelloHello");//false
  }
  public static void main(String[] args) {
    new Test().show();
  }
}

class Fruit{
  String color = "unknow";
  public Fruit getThis(){
    return this;
  }
  public void info(){
    System.out.println("fruit's method");
  }
}

public class Apple extends Fruit{

  String color = "red";//与父类同名的实例变量
  
  @Override
  public void info() {
    System.out.println("apple's method");
  }
  
  public void accessFruitInfo(){
    super.info();
  }
  public Fruit getSuper(){
    return super.getThis();
  }
  
  //for test purpose
  public static void main(String[] args) {
    Apple a = new Apple();
    Fruit f = a.getSuper();
    
    //Fruit f2 = a.getThis();
    //System.out.println(f == f2);//true
    
    System.out.println(a == f);//true
    System.out.println(a.color);//red
    System.out.println(f.color);//unknow
    
    a.info();//"apple's method"
    f.info();//"apple's method"
    
    a.accessFruitInfo();//"fruit's method"
  }
}

class Fruit{
    String color;
    
    public Fruit() {
      color = this.getColor();//父类color属性初始化依赖于重载的方法getColor
//      color = getColor();
    }
    public String getColor(){
      return "unkonw";
    }
    
    @Override
    public String toString() {
      return color;
    }
  }
  
  public class Apple extends Fruit{
  
    @Override
    public String getColor() {
      return "color: " + color;
    }
    
//    public Apple() {
//      color = "red";
//    }
    
    public static void main(String[] args) {
      System.out.println(new Apple());//color: null
    }
  }