如果类在java中有共同的字段，我如何使用它们的组合？

在Java中，如果多个类有共同的字段，可以通过以下几种方式使用它们的组合：

1. 继承（Inheritance）

通过继承，子类可以继承父类的字段和方法。

class CommonFields {
    protected String commonField1;
    protected int commonField2;

    public CommonFields(String commonField1, int commonField2) {
        this.commonField1 = commonField1;
        this.commonField2 = commonField2;
    }
}

class SubClass extends CommonFields {
    private String specificField;

    public SubClass(String commonField1, int commonField2, String specificField) {
        super(commonField1, commonField2);
        this.specificField = specificField;
    }

    public void display() {
        System.out.println("Common Field 1: " + commonField1);
        System.out.println("Common Field 2: " + commonField2);
        System.out.println("Specific Field: " + specificField);
    }
}

2. 组合（Composition）

通过组合，一个类可以包含另一个类的实例，从而共享字段。

class CommonFields {
    protected String commonField1;
    protected int commonField2;

    public CommonFields(String commonField1, int commonField2) {
        this.commonField1 = commonField1;
        this.commonField2 = commonField2;
    }
}

class SubClass {
    private CommonFields commonFields;
    private String specificField;

    public SubClass(String commonField1, int commonField2, String specificField) {
        this.commonFields = new CommonFields(commonField1, commonField2);
        this.specificField = specificField;
    }

    public void display() {
        System.out.println("Common Field 1: " + commonFields.commonField1);
        System.out.println("Common Field 2: " + commonFields.commonField2);
        System.out.println("Specific Field: " + specificField);
    }
}

3. 接口（Interface）

通过接口，可以定义一组共同的字段和方法，然后让多个类实现这个接口。

interface CommonFieldsInterface {
    String getCommonField1();
    int getCommonField2();
}

class SubClass implements CommonFieldsInterface {
    private String commonField1;
    private int commonField2;
    private String specificField;

    public SubClass(String commonField1, int commonField2, String specificField) {
        this.commonField1 = commonField1;
        this.commonField2 = commonField2;
        this.specificField = specificField;
    }

    @Override
    public String getCommonField1() {
        return commonField1;
    }

    @Override
    public int getCommonField2() {
        return commonField2;
    }

    public void display() {
        System.out.println("Common Field 1: " + getCommonField1());
        System.out.println("Common Field 2: " + getCommonField2());
        System.out.println("Specific Field: " + specificField);
    }
}