我们提供一个类：
class FooBar {
public void foo() {
for (int i = 0; i < n; i++) {
print("foo");
}
}
public void bar() {
for (int i = 0; i < n; i++) {
print("bar");
}
}
}
两个不同的线程将会共用一个 FooBar 实例。其中一个线程将会调用 foo() 方法，另一个线程将会调用 bar() 方法。
请设计修改程序，以确保 "foobar" 被输出 n 次。
示例 1:
输入: n = 1
输出: "foobar"
解释: 这里有两个线程被异步启动。其中一个调用 foo() 方法, 另一个调用 bar() 方法，"foobar" 将被输出一次。
示例 2:
输入: n = 2
输出: "foobarfoobar"
解释: "foobar" 将被输出两次。
思路1：synchronized
class FooBar {private int n;Object lock = new Object();boolean fooRun = false;public FooBar(int n) {this.n = n;}public void foo(Runnable printFoo) throws InterruptedException {for (int i = 0; i < n; i++) {synchronized(lock){while(fooRun){lock.wait();}// printFoo.run() outputs "foo". Do not change or remove this line.printFoo.run();fooRun = true;lock.notify();}}}public void bar(Runnable printBar) throws InterruptedException {for (int i = 0; i < n; i++) {synchronized(lock){while(!fooRun){lock.wait();}// printBar.run() outputs "bar". Do not change or remove this line.printBar.run();fooRun = false;lock.notify();}}}
}思路2：volatile
class FooBar {private int n;volatile boolean flag = false;public FooBar(int n) {this.n = n;}public void foo(Runnable printFoo) throws InterruptedException {for (int i = 0; i < n; i++) {while(flag){Thread.yield();}// printFoo.run() outputs "foo". Do not change or remove this line.printFoo.run();flag = true;}}public void bar(Runnable printBar) throws InterruptedException {for (int i = 0; i < n; i++) {while(!flag){Thread.yield();}// printBar.run() outputs "bar". Do not change or remove this line.printBar.run();flag = false;}}
}思路3：信号量
class FooBar {private int n;Semaphore foo_ = new Semaphore(1);Semaphore bar_ = new Semaphore(0);public FooBar(int n) {this.n = n;}public void foo(Runnable printFoo) throws InterruptedException {for (int i = 0; i < n; i++) {foo_.acquire();// printFoo.run() outputs "foo". Do not change or remove this line.printFoo.run();bar_.release();}}public void bar(Runnable printBar) throws InterruptedException {for (int i = 0; i < n; i++) {bar_.acquire();// printBar.run() outputs "bar". Do not change or remove this line.printBar.run();foo_.release();}}
}