Let's talk about instance initializers in Java

This article first gives an example to explore what is an instance variable initializer, what is an instance initializer, and what is a static initializer. Then look at how instance initializers work.

1. The order of execution

First look at the following code. Which part do you think will be executed first?

public class Foo {
  
    //instance variable initializer 实例变量初始化器
    String s = "abc";
  
    //constructor 构造函数
    public Foo() {
        System.out.println("constructor called");
    }
  
    //static initializer   静态初始化器
    static {
        System.out.println("static initializer called");
    }
  
    //instance initializer 实例变量初始化器
    {
        System.out.println("instance initializer called");
    }
  
    public static void main(String[] args) {
        new Foo();
        new Foo();
    }
}

The output is as follows:

static initializer called
instance initializer called
constructor called
instance initializer called
constructor called

2. How does Java's instance initializer work?

The instance initializer in the above example contains a code for printing characters. To understand how it works, we can imagine it as the process of variable assignment, so it is not difficult to understand.

In addition to assigning values ​​like this:

int b = 0;

You can also write:

int b;
b = 0;

Therefore, instance initializers and instance variable initializers are similar.

3. When is instance initializer used?

There are relatively few times when the strength initializer can be used. But sometimes there is an alternative to an instance variable initializer, for example:

• The initialization code must handle exceptions

• The execution of an instance variable fails Calculation by method

Of course, these requirements can still be implemented in the constructor. But if a class has multiple constructors, you have to repeat the same code in each constructor.

The code written in the instance initializer, no matter which constructor you use, the code in the instance initializer will be executed (it may be said more, but less used) ).

#Another application scenario for instance initializers is anonymous inner classes, because in this case no constructor can be created at all. (This might be a good place to place logging code?)

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