一、Object 类

如果一个类没有指定父类，默认就是继承 Object 类。Object 类里面共有 11 个方法：经常用到的 equals()，toString() 都是直接使用或者重写 Object 里面的这些方法；另外还有 final native 修饰的方法：反射需要用到的 getClass()，线程操作需要的 notify()，notifyAll()，wait(long timeout)；还有 native 修饰的方法：hashCode() 和 clone()

源码：

package java.lang;

public class Object {

    private static native void registerNatives();
    static {
        registerNatives();
    }

    public final native Class<?> getClass();

    public native int hashCode();

    public boolean equals(Object obj) {
        return (this == obj);
    }

    protected native Object clone() throws CloneNotSupportedException;

    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

    public final native void notify();

    public final native void notifyAll();

    public final native void wait(long timeout) throws InterruptedException;

    public final void wait(long timeout, int nanos) throws InterruptedException {
        if (timeout < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos > 0) {
            timeout++;
        }

        wait(timeout);
    }

    public final void wait() throws InterruptedException {
        wait(0);
    }

    protected void finalize() throws Throwable { }
}

二、Objects 类

Objects 类是 final 修饰的类，不可继承，内部方法都是 static 方法，从 jdk1.7 开始才引入了 Objects 类。

源码：

package java.util;

import java.util.function.Supplier;

public final class Objects {
    private Objects() {
        throw new AssertionError("No java.util.Objects instances for you!");
    }

    public static boolean equals(Object a, Object b) {
        return (a == b) || (a != null && a.equals(b));
    }

    public static boolean deepEquals(Object a, Object b) {
        if (a == b)
            return true;
        else if (a == null || b == null)
            return false;
        else
            return Arrays.deepEquals0(a, b);
    }

    public static int hashCode(Object o) {
        return o != null ? o.hashCode() : 0;
    }

    public static int hash(Object... values) {
        return Arrays.hashCode(values);
    }

    public static String toString(Object o) {
        return String.valueOf(o);
    }

    public static String toString(Object o, String nullDefault) {
        return (o != null) ? o.toString() : nullDefault;
    }

    public static <T> int compare(T a, T b, Comparator<? super T> c) {
        return (a == b) ? 0 :  c.compare(a, b);
    }

    public static <T> T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

    public static <T> T requireNonNull(T obj, String message) {
        if (obj == null)
            throw new NullPointerException(message);
        return obj;
    }

    public static boolean isNull(Object obj) {
        return obj == null;
    }

    public static boolean nonNull(Object obj) {
        return obj != null;
    }

    public static <T> T requireNonNull(T obj, Supplier<String> messageSupplier) {
        if (obj == null)
            throw new NullPointerException(messageSupplier.get());
        return obj;
    }
}