1 大纲
java.lang.Object是所有类的父类，默认继承，而且java.lang包下的所有类都由编译器自动导入，不需要显示import，因为用的多，提前加载可以提高运行时速度。
2 equals方法
"=="与equals的区别要看equals是如何重写的，在Object中两者意义等同，都是判断引用地址是否相同。在String中equals比较的是内容。
//Objectpublic boolean equals(Object obj) { return (this == obj);}//Stringpublic boolean equals(Object anObject) { if (this == anObject) { return true; } return (anObject instanceof String aString) && (!COMPACT_STRINGS || this.coder == aString.coder) && StringLatin1.equals(value, aString.value);}public static boolean equals(byte[] value, byte[] other) { if (value.length == other.length) { for (int i = 0; i < value.length; i++) { if (value[i] != other[i]) { return false; } } return true; } return false;}equals方法含义按需要重写，但需要满足Java规范：
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在Java规范中，对equals方法的使用必须遵循以下几个原则：1）自反性。对于任何非空引用值x，x. equals（x）都应返回true。2）对称性。对于任何非空引用值x和y，当且仅当y. equals（x）返回true时，x.equals（y）才应返回true。3）传递性。对于任何非空引用值x、y和z，如果x. equals（y）返回true，并且y.equals（z）返回true，那么x. equals（z）应返回true。4）一致性。对于任何非空引用值x和y，多次调用x. equals（y）始终返回true或始终返回false，前提是对象上equals比较中所用的信息没有被修改。对于任何非空引用值x，x. equals（null）都应返回false。对于任何非空引用值x，x. equals（null）都应返回false。
3 hashCode方法
hashCode是一个本地方法，用来加快equals比较，但两个不同对象的哈希值难免有冲突，hashCode和equals的关系如下：
如果equals返回true，则hashCode一定相等；如果equals返回false，则hashCode可能相等。也就是说如果hashCode不相等，那么equals一定不相等。注：Object中的hashCode方法返回的是对象的内存地址，有特殊要求可重写。
@IntrinsicCandidatepublic native int hashCode();hashCode主要用于Map、Set等容器中，当向容器添加元素时需要去一个个比较是否有相等的元素，直接调用equals效率太慢。可以先比较hashCode，如果hashCode不一样则equals必然返回false，如果hashCode一样再调用equals比较。
4 wait
wait方法也是Object类本地方法，一般用于synchronize代码块中，作用是释放锁并阻塞线程，唤醒方法是notify/notifyAll。
sleep方法是Thread类方法，调用了sleep0本地方法，作用是不释放锁但阻塞线程。
await方法是ConditionObject/ReentrantLock类的方法，作用是释放锁并阻塞线程，唤醒方法是signal/signalAll。
public final void wait() throws InterruptedException { wait(0L);}public final void wait(long timeoutMillis) throws InterruptedException { long comp = Blocker.begin(); try { wait0(timeoutMillis); } catch (InterruptedException e) { Thread thread = Thread.currentThread(); if (thread.isVirtual()) thread.getAndClearInterrupt(); throw e; } finally { Blocker.end(comp); }}// final modifier so method not in vtableprivate final native void wait0(long timeoutMillis) throws InterruptedException;public final void wait(long timeoutMillis, int nanos) throws InterruptedException { if (timeoutMillis < 0) { throw new IllegalArgumentException("timeoutMillis value is negative"); } if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException( "nanosecond timeout value out of range"); } if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) { timeoutMillis++; } wait(timeoutMillis);}public static void sleep(long millis) throws InterruptedException { if (millis < 0) { throw new IllegalArgumentException("timeout value is negative"); } if (currentThread() instanceof VirtualThread vthread) { long nanos = MILLISECONDS.toNanos(millis); vthread.sleepNanos(nanos); return; } if (ThreadSleepEvent.isTurnedOn()) { ThreadSleepEvent event = new ThreadSleepEvent(); try { event.time = MILLISECONDS.toNanos(millis); event.begin(); sleep0(millis); } finally { event.commit(); } } else { sleep0(millis); }}private static native void sleep0(long millis) throws InterruptedException;await具体细节请看Java多线程：条件变量
public final void await() throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); ConditionNode node = new ConditionNode(); int savedState = enableWait(node);//加入条件队列 LockSupport.setCurrentBlocker(this); // for back-compatibility，将AQS对象设置到thread中 boolean interrupted = false, cancelled = false, rejected = false; while (!canReacquire(node)) {//如果被唤醒进入同步队列后就可以跳出循环 if (interrupted |= Thread.interrupted()) { if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0) break; // else interrupted after signal } else if ((node.status & COND) != 0) { try { if (rejected) node.block(); else ForkJoinPool.managedBlock(node);//阻塞线程，最终会调用LockSupport.park() } catch (RejectedExecutionException ex) { rejected = true; } catch (InterruptedException ie) { interrupted = true; } } else Thread.onSpinWait(); // awoke while enqueuing }//被唤醒 LockSupport.setCurrentBlocker(null); node.clearStatus();////lock.lock()方法：acquire(null, arg, false, false, false, 0L);//重新获取锁时已原来的savedState acquire(node, savedState, false, false, false, 0L);//重新获取锁，此时该节点已经进入了同步队列，有可能直接tryAcquire成功跳出循环，也可能需要两次循环修改node.status为WAITING、park。 if (interrupted) { if (cancelled) { unlinkCancelledWaiters(node); throw new InterruptedException(); } Thread.currentThread().interrupt(); }}5 附录
public class Object { @IntrinsicCandidate public Object() {} @IntrinsicCandidate public final native Class getClass();//返回类对象用于反射 @IntrinsicCandidate public native int hashCode(); public boolean equals(Object obj) { return (this == obj); } @IntrinsicCandidate protected native Object clone() throws CloneNotSupportedException; public String toString() { return getClass().getName() + "@" + Integer.toHexString(hashCode()); } @IntrinsicCandidate public final native void notify(); @IntrinsicCandidate public final native void notifyAll(); public final void wait() throws InterruptedException { wait(0L); } public final void wait(long timeoutMillis) throws InterruptedException { long comp = Blocker.begin(); try { wait0(timeoutMillis); } catch (InterruptedException e) { Thread thread = Thread.currentThread(); if (thread.isVirtual()) thread.getAndClearInterrupt(); throw e; } finally { Blocker.end(comp); } } // final modifier so method not in vtable private final native void wait0(long timeoutMillis) throws InterruptedException; public final void wait(long timeoutMillis, int nanos) throws InterruptedException { if (timeoutMillis < 0) { throw new IllegalArgumentException("timeoutMillis value is negative"); } if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException( "nanosecond timeout value out of range"); } if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) { timeoutMillis++; } wait(timeoutMillis); } @Deprecated(since="9", forRemoval=true) protected void finalize() throws Throwable { }}关键词： Java JavaScript Node.js
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