java中 label 配合 break continue 使用的其實(shí)是比較少的.
這種做法在業(yè)務(wù)代碼中比較少見(jiàn).
在讀其他庫(kù)的代碼時(shí)可能會(huì)遇到該語(yǔ)法的使用.
下面對(duì)該語(yǔ)法做簡(jiǎn)短解釋.

break label

用來(lái)終止循環(huán)體的循環(huán),使用break 可以終止的循環(huán)體包括三種, for,while,do-while

不帶 label使用 break 時(shí)

執(zhí)行 break ,會(huì)終止離 break 關(guān)鍵字最近的循環(huán)體的循環(huán).
示例如下

for (int i = 0; i < 3; i++) {
    System.out.println(String.format("start outer for loop index %d", i));
    int j = 0;
    while (true) {
        System.out.println(String.format("inner loop with while index %d", j));
        if (j == 2) {
            //終止最近的for,while,do-while循環(huán)
            break;
        } else {
            j++;
        }
    }
    System.out.println(String.format("end outer for loop index %d", i));
}

結(jié)果為

start outer for loop index 0
inner loop with while index 0
inner loop with while index 1
inner loop with while index 2
end outer for loop index 0
start outer for loop index 1
inner loop with while index 0
inner loop with while index 1
inner loop with while index 2
end outer for loop index 1
start outer for loop index 2
inner loop with while index 0
inner loop with while index 1
inner loop with while index 2
end outer for loop index 2

通過(guò)打印可以看出,內(nèi)部循環(huán)每次執(zhí)行到2時(shí)就將當(dāng)前 while 死循環(huán)停止.
而外部循環(huán)則會(huì)從0遍歷到2,正常循環(huán)遍歷.不受 break 的影響.

帶 label 的break

會(huì)終止指定 label 所對(duì)應(yīng)的循環(huán)體.
示例為從二維的student age 數(shù)組中,找第一個(gè)年紀(jì)為9歲的學(xué)生.

int students[][] = {{10, 12, 11}, {8, 9, 1}, {9, 15, 7}};
jumpOutHere:
for (int i = 0; i < 3; i++) {
    System.out.println(String.format("start outer for loop index %d", i));
    int j = 0;
    while (j < 3) {
        System.out.println(String.format("current retrieve value %d", students[i][j]));
        if (students[i][j] == 9) {
            break jumpOutHere;
        } else {
            j++;
        }
    }
    System.out.println(String.format("end outer for loop index %d", i));
}

結(jié)果為

start outer for loop index 0
current retrieve value 10
current retrieve value 12
current retrieve value 11
end outer for loop index 0
start outer for loop index 1
current retrieve value 8
current retrieve value 9

當(dāng)遍歷到二維數(shù)組中第一個(gè)年紀(jì)為9的學(xué)生時(shí),直接終止 label 對(duì)應(yīng)的循環(huán).此處為外層循環(huán).

continue label

用來(lái)跳過(guò)循環(huán)體中本次執(zhí)行的循環(huán),使用continue 可以跳過(guò)的循環(huán)體包括三種, for,while,do-while

不帶 label 的 continue

會(huì)跳過(guò)距離continue最近的循環(huán)體的本次執(zhí)行,執(zhí)行下一次循環(huán)

for (int i = 0; i < 3; i++) {
    System.out.println(String.format("start outer for loop index %d", i));
    for(int k = 0;k<3;k++){
        if(k==1)
            continue;
        System.out.println(String.format("inner loop index %d",k));
    }
    System.out.println(String.format("end outer for loop index %d", i));
}

start outer for loop index 0
inner loop index 0
inner loop index 2
end outer for loop index 0
start outer for loop index 1
inner loop index 0
inner loop index 2
end outer for loop index 1
start outer for loop index 2
inner loop index 0
inner loop index 2
end outer for loop index 2

通過(guò)示例可以看出當(dāng) k=1時(shí)會(huì)跳過(guò)本次內(nèi)部循環(huán)(也是離 continue 最近的循環(huán)),直接執(zhí)行下一次內(nèi)部循環(huán).

帶 label 的 continue

會(huì)跳過(guò)標(biāo)記為 label 的循環(huán)體中的本次循環(huán)的執(zhí)行,并執(zhí)行, label 標(biāo)記位置的下一個(gè)循環(huán).

CONTINU_LABLE:
for (int i = 0; i < 3; i++) {
    System.out.println(String.format("start outer for loop index %d", i));
    for (int k = 0; k < 3; k++) {
        if (k == 1)
            continue CONTINU_LABLE;
        System.out.println(String.format("inner loop index %d", k));
    }
    System.out.println(String.format("end outer for loop index %d", i));
}

start outer for loop index 0
inner loop index 0
start outer for loop index 1
inner loop index 0
start outer for loop index 2
inner loop index 0

通過(guò)結(jié)果可以看出,當(dāng) k=1時(shí),直接就跳到CONTINU_LABLE標(biāo)記的循環(huán)體的下一次循環(huán)了.

結(jié)尾

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