equals(Object obj)有什么作用? [英] What does equals(Object obj) do?

问题描述

我经常在不同的地方找到一个equals方法.它实际上是做什么的?在每个班级都必须有这个重要吗?

I´ve often found an equals method in different places. What does it actually do? Is it important that we have to have this in every class?

   public boolean equals(Object obj)
    {
    if (obj == this)
    {
        return true;
    }
    if (obj == null)
    {
        return false;
    }
    if (obj instanceof Contact)
    {
        Contact other = (Contact)obj;
        return other.getFirstName().equals(getFirstName()) &&
                other.getLastName().equals(getLastName()) &&
                other.getHomePhone().equals(getHomePhone()) &&
                other.getCellPhone().equals(getCellPhone());

    }
    else
    {
        return false;
    }
}

推荐答案

它重新定义了对象的相等性".

It redefines "equality" of objects.

默认情况下(在java.lang.Object中定义)，仅当一个对象是同一实例时，该对象才等于另一个对象.但是当您覆盖它时，您可以提供自定义相等逻辑.

By default (defined in java.lang.Object), an object is equal to another object only if it is the same instance. But you can provide custom equality logic when you override it.

例如，java.lang.String通过比较内部字符数组来定义相等性.这就是为什么:

For example, java.lang.String defines equality by comparing the internal character array. That's why:

String a = new String("a"); //but don't use that in programs, use simply: = "a"
String b = new String("a");
System.out.println(a == b); // false
System.out.println(a.equals(b)); // true

即使您可能不需要测试是否相等，使用的类也可以.例如，List.contains(..)和List.indexOf(..)的实现使用.equals(..).

Even though you may not need to test for equality like that, classes that you use do. For example implementations of List.contains(..) and List.indexOf(..) use .equals(..).

检查 equals(..)方法所需的确切合同的javadoc .

Check the javadoc for the exact contract required by the equals(..) method.

在许多情况下，覆盖equals(..)时，您还必须覆盖hashCode()(使用相同的字段).这也在javadoc中指定.

In many cases when overriding equals(..) you also have to override hashCode() (using the same fields). That's also specified in the javadoc.

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