what this code is called

[jacob deiter]

Sorry to ask question on syntax, I tried to google but I did not get the clear answer. If you can help please help me.

What is this syntax called, what is the use of it.

Map<String,String> i=new TreeMap(){

// What is the purpose of this block?

}

[Campbell Ritchie]

It looks like the body of an anonymous class. Look in the directory where all your XYZ.class files are, and you will probably find one called PQR$1.class or similar.

[Junilu Lacar]

The right hand side of that assignment statement is an anonymous class, formally defined in the Java Language Specification as an UnqualifiedClassInstanceCreationExpression: https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.9

UnqualifiedClassInstanceCreationExpression:
new [TypeArguments] ClassOrInterfaceTypeToInstantiate ( [ArgumentList] ) [ClassBody]

The purpose of the block is to provide a one-off implementation of a method or several methods of the declared class type. In your example, you're basically telling Java to "Create an instance of a new subclass of TreeMap with the following implementations of these methods (defined inside the block)."

[Stephan van Hulst]

This is an "anonymous class declaration". The block is used to define the class body of an anonymous custom type that extends TreeMap. It's as if you had written the following code:

class MyTreeMap extends TreeMap {

  // What is the purpose of this block?

}

Map<String, String> map = new MyTreeMap();

The important difference is that the class that you declare using an anonymous class declaration doesn't have a name, and a new instance of that class is created instantly.

Anonymous classes are used when you don't need to refer to the custom type outside of the method, and you only need one instance per method call. They're usually not used to extend complete classes like you did with TreeMap, but instead they are usually used to implement interfaces or abstract classes. The most common use case are event listeners:

panel.addMouseListener(new MouseAdapter() {
  @Override
  public void mouseClicked(MouseEvent event) {
    drawText(panel, "Hello World!", event.getPoint());
  }
});

Many interfaces have only one abstract method, and since Java 8 you can implement them using a lambda expression instead of an anonymous class declaration:

button.addActionListener(event -> textArea.append(String.format("%n> Hello World!")));

However, anonymous class declarations are still useful if an interface consists of more than one abstract method, or when you want to implement an abstract class, like I did above with MouseAdapter.

Some people abuse anonymous class declarations to add some initialization code to the new instance:

var map = new TreeMap<String, String>() {{
  put("First name", "Stephan"  );
  put("Last name" , "van Hulst");
}};

This syntax looks super weird, but it's really as if you wrote the following class:

class MyTreeMap extends TreeMap {

  // This is an initializer block. It's similar to a constructor.
  {
    put("First name", "Stephan"  );
    put("Last name" , "van Hulst");
  }
}

var map = new MyTreeMap();

Do NOT do this. It's poor, lazy programming. Instead, do this:

var map = new TreeMap(Map.of(
    "First name", "Stephan",
    "Last name" , "van Hulst"
));

[Paul Clapham]

Stephan van Hulst wrote:Do NOT do this. It's poor, lazy programming. Instead, do this:

var map = new MyTreeMap(Map.of(
    "First name", "Stephan",
    "Last name" , "van Hulst"
));

This is certainly true since Java 9, when Map.of arrived. So yeah, you should be using Map.of in new code now. But you're likely to find the double-brace hack in code which was written before then.

[jacob deiter]

Thank you!!!