JAVA8跳出循环-forEach实现continue、break效果

概述

我们都知道continue、break用在哪里，怎么用以及用来干啥，即

continue

• 用于循环体内

• 用来结束本次循环
  break

• 用于循环体内

• 用来跳出循环
  但是，forEach有些特殊，上面的关键字并不能在forEach循环体内直接用，否则会编译不通过，具体报错为

break outside switch or loop或
continue outside of loop

普通循环

fori循环

collection、map、Map.Entry的循环也类似，这里不再赘述

//example 1
int[] intArr = new int[]{1,2,3,4,5,6,7,8,9,10};
StringBuilder content =  new StringBuilder(16);
for (int i = 0; i < intArr.length; i++) {
    int item = intArr[i];
    //跳出本次循环，继续下一轮循环
    if(item == 5){ continue;}
    //结束循环
    if(item == 8){ break;}
    content.append(item);
}
Assert.assertEquals("哦豁，写bug了吧","123467",content.toString());

for循环

collection、map、Map.Entry的循环也类似，这里不再赘述

//example2
int[] intArr = new int[]{1,2,3,4,5,6,7,8,9,10};
StringBuilder content = new StringBuilder(16);
for (int item : intArr) {
    //跳出本次循环，继续下一轮循环
    if(item == 5){ continue;}
    //结束循环
    if(item == 8){ break;}
    content.append(item);
}
Assert.assertEquals("哦豁，写bug了吧","123467",content.toString());

forEach循环

Iterable.forEach循环 —— by MutableBoolean

//example 3
//注1 用continue会报错，并提示：continue outside of loop
//注2 用break会报错，并提示：break outside switch or loop
//注1 return实现continue的效果
//注2 用try-catch实现break效果
Collection<Integer> c = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
StringBuilder content = new StringBuilder(16);
try {
    c.forEach((item) -> {
        if (item == 5) {return;}
        if (item == 8) {
            throw new RuntimeException("不用再继续循环了");
        }else{
           content.append(item);
        }
    });
} catch (RuntimeException ex) {
}
Assert.assertEquals("哦豁，写bug了吧","123467",content.toString());

用try-catch这种抛异常的方式来跳出循环确实太挫了(代码不简洁不美观、且使用场景不太对，毕竟异常是意料之外，而这个跳出循环是正常的一种程序应用场景)，在看看下面这种MutableBoolean方式

//example 4
//collection: MutableBoolean来实现break功能; return实现continue
//注1 MutableBoolean的定义方式见附录
Collection<Integer> c = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
StringBuilder content = new StringBuilder(16);
final MutableBoolean mutableBoolean = new MutableBoolean(true);
c.forEach((item) -> {
    if (item == 5) {return;}
    if (item == 8) {
        mutableBoolean.setFlag(Boolean.FALSE);
    }else if(mutableBoolean.getFlag()) {
        content.append(item);
    }
});
Assert.assertEquals("哦豁，写bug了吧","123467",content.toString());

see 是不是elegant、简洁多了。在看看Map的方式，其实也是一模一样的

StringBuilder content = new StringBuilder(16);
final MutableBoolean mutableBoolean = new MutableBoolean(true);
Map<Integer,Integer> map = new HashMap<>(16);
for (int i = 0; i <10 ; i++) {
    map.put(i+1,i+1);
}
map.forEach((k,v)->{
    if (v == 5) {return;}
    if (v == 8) {
        mutableBoolean.setFlag(Boolean.FALSE);
    }else if(mutableBoolean.getFlag()) {
        content.append(v);
    }
});
Assert.assertEquals("哦豁，写bug了吧","123467",content.toString());

Stream.forEach —— by anyMatch

一种是anyMatch

//example 6
//stream: anyMatch跳出循环; return false 实现continue
Collection<Integer> c = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
StringBuilder content = new StringBuilder(16);
c.stream().anyMatch((item)->{
    if (item == 5) {return false;}
    boolean flag =  8 == item;
    if(!flag) {
        content.append(item);
    }
    return flag;
}) ;
Assert.assertEquals("哦豁，写bug了吧","123467",content.toString());

Stream.forEach —— by findAny

//example 7
//stream: findAny跳出循环; return false 实现continue
Collection<Integer> c = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
StringBuilder content = new StringBuilder(16);
c.stream().filter((item)->{
    if (item == 5) {return false;}
    boolean flag =  8 == item;
    if(!flag) {
        content.append(item);
    }
    return flag;
}).findAny() ;
Assert.assertEquals("哦豁，写bug了吧","123467",content.toString());

附录

MutableBoolean

/**
 * 可变布尔变量
 */
static class MutableBoolean{
    //标记
    private Boolean flag;
    public MutableBoolean(Boolean flag) {
        this.flag = flag;
    }
    public Boolean getFlag() {
        return flag;
    }
    public void setFlag(Boolean flag) {
        this.flag = flag;
    }
}

参考

• Java8中的foreach跳出循环break/return