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What is an initialization block?

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青春惊慌失措
青春惊慌失措 2020-11-22 03:42

We can put code in a constructor or a method or an initialization block. What is the use of initialization block? Is it necessary that every java program must have it?

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  • 被撕碎了的回忆
    被撕碎了的回忆 (楼主)
    2020-11-22 03:49

    The question is not entirely clear, but here's a brief description of ways you can initialise data in an object. Let's suppose you have a class A that holds a list of objects.

    1) Put initial values in the field declaration:

    class A {
    private List data = new ArrayList();
}



    2) Assign initial values in the constructor:
    


    class A {
    private List data;
    public A() {
        data = new ArrayList();
    }
}



    These both assume that you do not want to pass "data" as a constructor argument.
    


    Things get a little tricky if you mix overloaded constructors with internal data like above. Consider:
    


    class B {
    private List data;
    private String name;
    private String userFriendlyName;

    public B() {
        data = new ArrayList();
        name = "Default name";
        userFriendlyName = "Default user friendly name";
    }

    public B(String name) {
        data = new ArrayList();
        this.name = name;
        userFriendlyName = name;
    }

    public B(String name, String userFriendlyName) {
        data = new ArrayList();
        this.name = name;
        this.userFriendlyName = userFriendlyName;
    }
}



    Notice that there is a lot of repeated code. You can fix this by making constructors call each other, or you can have a private initialisation method that each constructor calls:
    


    class B {
    private List data;
    private String name;
    private String userFriendlyName;

    public B() {
        this("Default name", "Default user friendly name");
    }

    public B(String name) {
        this(name, name);
    }

    public B(String name, String userFriendlyName) {
        data = new ArrayList();
        this.name = name;
        this.userFriendlyName = userFriendlyName;
    }
}



    or
    


    class B {
    private List data;
    private String name;
    private String userFriendlyName;

    public B() {
        init("Default name", "Default user friendly name");
    }

    public B(String name) {
        init(name, name);
    }

    public B(String name, String userFriendlyName) {
        init(name, userFriendlyName);
    }

    private void init(String _name, String _userFriendlyName) {
        data = new ArrayList();
        this.name = name;
        this.userFriendlyName = userFriendlyName;
    }
}



    The two are (more or less) equivalent.
    


    I hope that gives you some hints on how to initialise data in your objects. I won't talk about static initialisation blocks as that's probably a bit advanced at the moment.
    


    EDIT: I've interpreted your question as "how do I initialise my instance variables", not "how do initialiser blocks work" as initialiser blocks are a relatively advanced concept, and from the tone of the question it seems you're asking about the simpler concept. I could be wrong.
    
    
    
             
    
                                                        
    
            
            
    
              
                
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