# LeetCode 1115. Print FooBar Alternately--多线程并发问题--Java解法--CyclicBarrier, synchronized, Semaphore 信号量

2020/01/12

Suppose you are given the following code:

``````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");
}
}
}
``````

The same instance of FooBar will be passed to two different threads. Thread A will call foo() while thread B will call bar(). Modify the given program to output “foobar” n times.

Example 1:

``````Input: n = 1
Output: "foobar"
Explanation: There are two threads being fired asynchronously. One of them calls foo(), while the other calls bar(). "foobar" is being output 1 time.
``````

Example 2:

``````Input: n = 2
Output: "foobarfoobar"
Explanation: "foobar" is being output 2 times.
``````

Java解法如下：

``````class FooBar {
volatile private int n;
private int flag = 0;

public FooBar(int n) {
this.n = n;
}

public void foo(Runnable printFoo) throws InterruptedException {
for (int i = 0; i < n; i++) {
synchronized (this) {
while (flag == 1) {
this.wait();
}
// printFoo.run() outputs "foo". Do not change or remove this line.
printFoo.run();
flag = 1;
this.notifyAll();
}
}
}

public void bar(Runnable printBar) throws InterruptedException {
for (int i = 0; i < n; i++) {
synchronized (this) {
while (flag == 0) {
this.wait();
}
// printBar.run() outputs "bar". Do not change or remove this line.
printBar.run();
flag = 0;
this.notifyAll();
}
}
}
}
``````

``````
class FooBar {
private int n;
private final CyclicBarrier barrier = new CyclicBarrier(2);

public FooBar(int n) {
this.n = n;
}

public void foo(Runnable printFoo) throws InterruptedException {

for (int i = 0; i < n; i++) {
// printFoo.run() outputs "foo". Do not change or remove this line.
printFoo.run();
try {
barrier.await();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}

public void bar(Runnable printBar) throws InterruptedException {
for (int i = 0; i < n; i++) {
try {
barrier.await();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
// printBar.run() outputs "bar". Do not change or remove this line.
printBar.run();
}
}
}
``````

``````class FooBar {
private int n;
private final Semaphore foo = new Semaphore(0);
private final Semaphore bar = new Semaphore(1);

public FooBar(int n) {
this.n = n;
}

public void foo(Runnable printFoo) throws InterruptedException {

for (int i = 0; i < n; i++) {
bar.acquire();
// printFoo.run() outputs "foo". Do not change or remove this line.
printFoo.run();
foo.release();
}
}

public void bar(Runnable printBar) throws InterruptedException {
for (int i = 0; i < n; i++) {
foo.acquire();
// printBar.run() outputs "bar". Do not change or remove this line.
printBar.run();
bar.release();
}
}
}
``````