在Java中覆盖equals方法

考虑以下Java程序：

Java

class Complex {
    private double re, im;   
     
    public Complex(double re, double im) {
        this.re = re;
        this.im = im;
    }
}
 
// Driver class to test the Complex class
public class Main {
    public static void main(String[] args) {
        Complex c1 = new Complex(10, 15);
        Complex c2 = new Complex(10, 15);
        if (c1 == c2) {
            System.out.println("Equal ");
        } else {
            System.out.println("Not Equal ");
        }
    }
}

输出：

Not Equal 

打印“Not Equal”的原因很简单：当我们比较 c1 和 c2 时，检查 c1 和 c2 是否引用同一个对象（对象变量在Java中总是引用）。 c1 和 c2 指的是两个不同的对象，因此值 (c1 == c2) 为 false。如果我们创建另一个引用说 c3 如下所示，那么 (c1 == c3) 将给出 true。

Java

Complex c3 = c1;  // (c3 == c1) will be true

那么，我们如何检查对象内部的值是否相等呢？ Java中的所有类都直接或间接地从 Object 类继承（参见第 1 点）。 Object 类有一些基本方法，如 clone()、toString()、equals() 等。我们可以重写类中的 equals 方法来检查两个对象是否具有相同的数据。

Java

class Complex {
 
    private double re, im;
 
    public Complex(double re, double im) {
        this.re = re;
        this.im = im;
    }
 
    // Overriding equals() to compare two Complex objects
    @Override
    public boolean equals(Object o) {
 
        // If the object is compared with itself then return true 
        if (o == this) {
            return true;
        }
 
        /* Check if o is an instance of Complex or not
          "null instanceof [type]" also returns false */
        if (!(o instanceof Complex)) {
            return false;
        }
         
        // typecast o to Complex so that we can compare data members
        Complex c = (Complex) o;
         
        // Compare the data members and return accordingly
        return Double.compare(re, c.re) == 0
                && Double.compare(im, c.im) == 0;
    }
}
 
// Driver class to test the Complex class
public class Main {
 
    public static void main(String[] args) {
        Complex c1 = new Complex(10, 15);
        Complex c2 = new Complex(10, 15);
        if (c1.equals(c2)) {
            System.out.println("Equal ");
        } else {
            System.out.println("Not Equal ");
        }
    }
}

输出：

Equal