Java -- 增强for循环(foreach)

增强for循环

相对于经典for循环, foreach可以减少代码量，但不是所有情况下foreach都可以代替for循环

当需要修改元素的值或和下标相关的操作需要使用标准for循环

foreach格式

for (数组元素类型 临时变量: 遍历的对象) {}

临时变量代表的是数组的元素,而非下标

foreach对对象进行只读访问, 具有一定的安全性, 因此对数组/集合遍历时优选增强for循环

// 经典for循环
import java.util.Random;

int[] arr = new int[5];
Random r = new Random();
for (int i = 0; i < arr.length; i++) {
	arr[i] = r.nextInt(100) + 1;
}
for (int i = 0; i < arr.length; i++) {
	int tem = arr[i];
	tmp *= 10;
	System.out.print(tmp + " ");
}
System.out.println();

以上代码可简化为:

int[] arr = new int[5];
Random r = new Random;
for (int i = 0; i < arr.length; i++) {
	arr[i] = r.nextInt(100) + 1;
}
for (int i: arr2) {
	System.out.print((i *= 10) + " ");
}

找出数组中的最值并求和

int max, min, sum;
max = 0x80000000;
min = 0x7FFFFFFF;
sum = 0;
for (int i: arr) {
	max = max > i? max: i;
	min = min < i? min: i;
	sum += i;
}
System.out.println("max: " + max + " min: " + min + " sum: " + sum);

删除列表中指定下标的元素, 并缩减数组

// Person.java
public class Person {
    private String name;
    private int age;

    public Person() {
    }

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String say(Person person) {
        return person.name + " " + person.age;
    }
}

// Student.java
public class Student extends Person{
    private String sno;

    public Student() {}

    public Student(String sno) {
        this.sno = sno;
    }

    public Student(String sno, String name, int age) {
        super(name, age);
        this.sno = sno;
    }

    public String getSno() {
        return sno;
    }

    public void setSno(String sno) {
        this.sno = sno;
    }

    public void say() {
        String str = super.say(this);
        System.out.println(this.sno + " " + str);
    }
}

// Demo.java
Student[] studentArr = new Student[5];
        studentArr[0] = new Student("001", "小一", 17);
        studentArr[1] = new Student("002", "大二", 18);
        studentArr[2] = new Student("003", "张三", 19);
        studentArr[3] = new Student("004", "李四", 17);
        studentArr[4] = new Student("005", "王五", 18);
int index = 2;
int count = studentArr.length - 1;
// 将删除元素的右侧所有元素左移
for (int i = index; i < studentArr.length - 1; i++) {
	studentArr[i] = studentArr[i + 1];
}
// 最后一个元素置为null
studentArr[studentArr.length - 1] = null;
// 调用student的say方法
for (Student student: studentArr) {
	if (student != null){
		student.say();
	} else {
		System.out.println(student);
	}
}
System.out.println("=====================");

// 缩减数组
Student[] studentArray = new Student[count];
for (int i = 0; i < studentArray.length; i++) {
	studentArray[i] = studentArr[i];
}

// 将旧数组指向新数组地址
studentArr = studentArray;

for (Student student: studentArr) {
	student.say();
}