java isAssignableFrom 用法

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    /**     * Determines if the class or interface represented by this     * {@code Class} object is either the same as, or is a superclass or     * superinterface of, the class or interface represented by the specified     * {@code Class} parameter. It returns {@code true} if so;     * otherwise it returns {@code false}. If this {@code Class}     * object represents a primitive type, this method returns     * {@code true} if the specified {@code Class} parameter is     * exactly this {@code Class} object; otherwise it returns     * {@code false}.     *     * <p> Specifically, this method tests whether the type represented by the     * specified {@code Class} parameter can be converted to the type     * represented by this {@code Class} object via an identity conversion     * or via a widening reference conversion. See <em>The Java Language     * Specification</em>, sections 5.1.1 and 5.1.4 , for details.     *     * @param cls the {@code Class} object to be checked     * @return the {@code boolean} value indicating whether objects of the     * type {@code cls} can be assigned to objects of this class     * @exception NullPointerException if the specified Class parameter is     *            null.     * @since JDK1.1     */    public native boolean isAssignableFrom(Class<?> cls);

A instanceof B 相比大家都很熟悉，判断A对象是否为B类或接口的实例或者子类或子接口的实例，

A isAssignableFrom B 是Class类的一个native方法，很多时候没直接用到，用Class对象的时候才会用到，A Class对象所代表的类或者接口是否为B Class对象所代表的类或者接口，或者B Class对象所有代表的类或者接口的子类或子接口，是的话就返回true否则false。

听起来好拗口，确实，下面用代码例子来解释

    class A {    }    class B extends A{    }    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        Log.d(TAG, B.class.isAssignableFrom(A.class) + "");        Log.d(TAG, B.class.isAssignableFrom(B.class) + "");        Log.d(TAG, A.class.isAssignableFrom(A.class) + "");        Log.d(TAG, A.class.isAssignableFrom(B.class) + "");        Log.d(TAG, new A().getClass().isAssignableFrom(A.class) + "");        Log.d(TAG, new A().getClass().isAssignableFrom(B.class) + "");        Log.d(TAG, new B().getClass().isAssignableFrom(B.class) + "");        Log.d(TAG, new B().getClass().isAssignableFrom(A.class) + "");    }

因为我是直接在安卓项目里面测试的，所以有个onCreate()，改成纯java的对大家来说应该都不是事，忽略这个吧

看下打印结果

12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: false
12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: true
12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: true
12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: true
12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: true
12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: true
12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: true
12-04 08:57:56.747 21395-21395/android.example.com.customannotationdemo D/MainActivity: false

相信不用我解释，看到这个结果大家应该都明白了

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