JAVA基础之五：流程控制

1. 前言

2. 顺序流控制

3. 条件流控制

语法	例子	流程图
// if-then if ( booleanExpression ) { true-block ; }	if (mark >= 50) { System.out.println("恭喜!"); System.out.println("坚持!"); }
// if-then-else if ( booleanExpression ) { true-block ; } else { false-block ; }	if (mark >= 50) { System.out.println("恭喜!"); System.out.println("坚持!"); } else { System.out.println("努力!"); }
if ( booleanExpr-1 ) { block-1 ; } else if ( booleanExpr-2 ) { block-2 ; } else if ( booleanExpr-3 ) { block-3 ; } else if ( booleanExpr-4 ) { ...... } else { elseBlock ; }	if (mark >= 80) { System.out.println("A"); } else if (mark >= 70) { System.out.println("B"); } else if (mark >= 60) { System.out.println("C"); } else if (mark >= 50) { System.out.println("D"); } else { System.out.println("F"); }
// switch-case-default switch ( selector ) { case value-1: block-1; break; case value-2: block-2; break; case value-3: block-3; break; ...... case value-n: block-n; break; default: default-block; }	char oper; int num1, num2, result; ...... switch (oper) { case '+': result = num1 + num2; break; case '-': result = num1 - num2; break; case '*': result = num1 * num2; break; case '/': result = num1 / num2; break; default: System.err.println("error"); }

if (mark >= 50)
System.out.println("通过");   // 不推荐这样使用
else {
System.out.println("失败");
System.out.println("努力!");
}

booleanExpr ? trueExpr : falseExpr

System.out.println((mark >= 50) ? "PASS" : "FAIL");
max = (a > b) ? a : b;   // RHS返回a或者b
abs = (a > 0) ? a : -a;  // RHS返回a或者-a

4. 循环控制流

语法	例子	流程图
// for-loop for (initialization; test; post-processing){ body; }	// 从1加到1000 int sum = 0; for (int number = 1; number <= 1000; ++number) { sum += number; }
// while-do loop while ( test ) { body; }	int sum = 0, number = 1; while (number <= 1000) { sum += number; ++number; }
// do-while loop do { body; } while(test);	int sum = 0, number = 1; do { sum += number; ++number; } while (number <= 1000);

boolean valid = true;
int number;
do {
// 你可以提示用户输入1到10
......
valid = number < 1 || number > 10;
} while (valid);

// 使用while-do的例子
boolean valid = true;
while(valid){
// 你可以提示用户输入1到10
......
valid = number < 1 || number > 10;
}

for (int row = 0, col = 0; row < SIZE; ++row, ++col) {
......
}

5. break和continue

public class NonPrimeList {
public static void main(String[] args) {
int upperbound = 100;

for (int number = 2; number <= upperbound; ++number) {
// 非素数可以整除2和该数平方根之间的一个数
int maxFactor = (int)Math.sqrt(number);
for (int factor = 2; factor <= maxFactor; ++factor) {
if (number % factor == 0) {
System.out.println(number + "是一个非素数");
break;   // 找到一个整除的数即可
}
}
}
}
}

public class PrimeListWithBreak {
public static void main(String[] args) {
int upperbound = 100;

for (int number = 2; number <= upperbound; ++number) {
int maxFactor = (int)Math.sqrt(number);
boolean isPrime = true;  // 假设当前为素数
for (int factor = 2; factor <= maxFactor; ++factor) {
if (number % factor == 0) {
isPrime = false;
break;
}
}

if (isPrime) System.out.println(number + "是一个素数");
}
}
}

public class PrimeList {
public static void main(String[] args) {
int upperbound = 100;

for (int number = 2; number <= upperbound; ++number) {
int maxFactor = (int)Math.sqrt(number);
boolean isPrime = true;
int factor = 2;
while (isPrime && factor <= maxFactor) {
if (number % factor == 0) {
isPrime = false;
}
++factor;
}

if (isPrime) System.out.println(number + "是一个素数");
}
}
}

/*
* 从1累加到上届，11, 22, 33,...
*/
int upperbound = 100;
int sum = 0;

for (int number = 1; number <= upperbound; ++number) {
if (number % 11 == 0) continue;  // 跳过剩下的语句，继续进行下一次迭代
sum += number;
}

// 这样写更好
for (int number = 1; number <= upperbound; ++number) {
if (number % 11 != 0) sum += number;
}

public class MysterySeries {
public static void main(String[] args) {
int number = 1;

while(true) {
++number;
if ((number % 3) == 0) continue;
if (number == 133) break;
if ((number % 2) == 0) {
number += 3;
} else {
number -= 3;
}
System.out.print(number + " ");
}
}
}

level1:                             // 定义level1循环标签
for (.....) {
level2:                         // 定义level2循环标签
for (.....) {
for (......) {              // level3循环标签
if (...) break level1;  // 跳出所有循环
if (...) break level2:  // 跳出level2循环，继续下一次level1循环
......
}
}
}

level1:
for (.....) {
level2:
for (.....) {
for (......) {
if (...) continue level1;  // 马上执行下一次level1循环
if (...) continue level2:  // 马上执行下一次level2循环
......
}
}
}

6. 终止程序

if (errorCount > 10) {
System.out.println("太多的异常");
System.exit(1);  // 异常终止
}

public static void main(String[] args) {
...
if (errorCount > 10) {
System.out.println("too many errors");
return;
}
...
}

7. 嵌套循环

# # # # # # # #
# # # # # # # #
# # # # # # # #
# # # # # # # #
# # # # # # # #
# # # # # # # #
# # # # # # # #
# # # # # # # #

public class PrintSquarePattern {
public static void main(String[] args) {
int size = 8;
for (int row = 1; row <= size; ++row) {      // 打印行
for (int col = 1; col <= size; ++col) {  // 打印列
System.out.print("# ");
}
System.out.println();                    // 下一行
}
}
}

8. 控制流中的一些问题

if (i == 0)
if (j == 0)
System.out.println("i和j都是0");
else
System.out.println("i不是0");   // 本打算匹配最外层的if

if ( i == 0) {
if (j == 0) System.out.println("i和j都是0");
} else {
System.out.println("i不是0");   // 明确的匹配外层if
}

while (true) { ...... }