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。

(资料图片仅供参考)


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();
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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);}
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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;
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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();    }}
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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 { }}
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