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;
}
}
