如何在java中设置公共静态最终类实例的值

Question

example

public static final class MyClass{
        private long id_;
        public static final int MEMBER_ID_FIELD_NUMBER = 2;
        private long startDate_;
    ....

    }

//meanwhile in another class
class Abc {
        ...
    public final void aFunc {
        MyClass aaa = ???? // here is what I want to know how to set the class values
    ....
    }

}

How can I set the value for MyClass in the other class? I tried to do "initialize" like

MyClass aaa = {
                id_: 123123132,...
            };

it just give me error that id_ `can't resolve symbol.

I come from javascript background, that's why I think like this. Any idea how can I initiate the instance with partial / all custom default values for a public static final class ? Btw, if the back-story can help get a full picture --- I do this for testing a specific class's method which is including an instance of MyClass . This is an attempt to mock the values instead having to recreate a whole ActionBean to just test one method.

Answer 1

If it has default constructor (the one without any argument) you can first create object without providing fields values and the set it with setters (if they exist) like so:

public final void aFunc {
        MyClass aaa = new MyClass();
        a.setId_(1);
        ...
}

If it doesn't have default constructor you need to provide all parameters during object creation, like so:

public final void aFunc {
        MyClass aaa = new MyClass(1,2,3);
        ....
}

Answer 2

There are multiple ways to create an object instance and assign values to fields:

1. Call setter methods:

    public static final class MyClass { private long id_; public void setId(long id) { this.id_ = id; } }

    MyClass aaa = new MyClass(); aaa.setId(123123132);

2. Use a constructor with arguments:

    public static final class MyClass { private long id_; public MyClass(long id) { this.id_ = id; } }

    MyClass aaa = new MyClass(123123132);

3. Use public fields. Although not recommended, this is what JavaScript kind of does:

    public static final class MyClass { public long id_; }

    MyClass aaa = new MyClass(); aaa.id_ = 123123132;

4. Use a builder:

    public static final class MyClass { private long id_; private MyClass(long id) { this.id_ = id; } public static class Builder { private long id_; public void id(long id) { this.id_ = id; } public MyClass build() { MyClass x = new MyClass(); x.id_ = this.id_; return x; } } }

    MyClass aaa = new MyClass.Builder() .id(123123132) .build();

Answer 3

You have two options in order to fill the fields with values. One options is using the public constructor passing all the parameters you want to update:

public static final class MyClass{
    private long id_;
    public static final int MEMBER_ID_FIELD_NUMBER = 2;
    private long startDate_;

    public MyClass(long id_, long startDate_) {
        this.id_ = id_;
        this.startDate_ = startDate_;
    }
}

You can use it as bellow:

public final void aFunc {
    MyClass objectA = new MyClass(1L, 23214112213L);
}

Second option is using setters: method per field to add value:

public static final class MyClass{
    private long id_;
    public static final int MEMBER_ID_FIELD_NUMBER = 2;
    private long startDate_;
    
    public void setID(long id_) {
         this.id_ = id_;
    }

    public void setStartDate(long startDate_) {
         this.startDate_ = startDate_;
    }
        
}

You can use them as bellow:

public final void aFunc {
    MyClass objectB = new MyClass();
    objectB.setID(1L);
    objectB.setStartDate(23214112213L);
}

But you can combine them. You can have the constructor and bellow the setters.

See java constructor and see setters and getters

• For you private fields i assume you need getters too.

Those are part of the basic knowledge you should have. In addition keep in mind there are conventions (naming etc)