【我的Java笔记】增强for循环 & ArrayList集合储存自定义对象并遍历的五种方法

增强for循环

1.增强for循环的作用：

在集合中，增强for循环的作用是用来代替迭代器

2.优点：书写格式简单，代码量少

3.弊端：遍历数组或集合的时候，数组对象或者集合对象不能为null（需加上非空判断）

注：在Java中，只要是对象，那么一定要给对象做非空判断，避免出现空指针异常

4.书写格式：

for(数组或者集合中的数据类型  变量名：数组或者集合对象)｛

变量的输出；

｝

例：

import java.util.ArrayList;

public class Test2 {
public static void main(String[] args) {
//创建ArrayList集合对象
ArrayList<String> al = new ArrayList<String>();
//给集合中添加元素
al.add("hello");
al.add("world");
al.add("java");

//给集合对象进行非空判断
if(al!=null) {
//使用增强for循环遍历集合
for(String str:al) {
System.out.println(str);
}
}

}
}

ArrayList集合储存自定义对象并遍历的五种方法

1.使用集合遍历（toArray()方法）：

import java.util.ArrayList;

//测试类
public class StudentTest {
public static void main(String[] args) {
//创建学生类对象
Student s1 = new Student("国际米兰",14);
Student s2 = new Student("罗马",56);
Student s3 = new Student("贝尔格莱德",45);

//创建ArrayList集合对象
ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);

//使用toArray()方法将集合转为数组
Object [] obj = al.toArray();
//遍历数组
for(int i=0;i<obj.length;i++) {
//将对象obj[i]向下转型
Student s = (Student)obj[i];
//输出
System.out.println(s);
}
}
}

//学生类
class Student{
private String name;
private int age;

public Student() {
super();
}

public Student(String name, int age) {
super();
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;
}

@Override
public String toString() {
return "["+name+","+age+"]";
}

}

2.Collection集合的迭代器进行集合遍历（Iterator iterator()方法）

import java.util.ArrayList;
import java.util.Iterator;

//测试类
public class StudentTest {
public static void main(String[] args) {

System.out.println("使用Collection集合的迭代器Iterator iterator()方法进行集合遍历的结果：");

//创建学生类对象
Student s1 = new Student("国际米兰",14);
Student s2 = new Student("罗马",56);
Student s3 = new Student("贝尔格莱德",45);

//创建ArrayList集合对象
ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);

//获取迭代器对象
Iterator<Student> it = al.iterator();
while(it.hasNext()) {
Student s = it.next();
System.out.println(s);
}
}
}

//学生类
class Student{
private String name;
private int age;

public Student() {
super();
}

public Student(String name, int age) {
super();
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;
}

@Override
public String toString() {
return "["+name+","+age+"]";
}

}

3.List集合的迭代器进行遍历（ListIterator listIterator()方法）

import java.util.ArrayList;
import java.util.ListIterator;

//测试类
public class StudentTest {
public static void main(String[] args) {

System.out.println("使用List集合的迭代器ListIterator listIterator()方法进行集合遍历的结果：");

//创建学生类对象
Student s1 = new Student("国际米兰",14);
Student s2 = new Student("罗马",56);
Student s3 = new Student("贝尔格莱德",45);

//创建ArrayList集合对象
ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);

//获取迭代器对象
ListIterator<Student> it = al.listIterator();
while(it.hasNext()) {
Student s = it.next();
System.out.println(s);
}
}
}

//学生类
class Student{
private String name;
private int age;

public Student() {
super();
}

public Student(String name, int age) {
super();
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;
}

@Override
public String toString() {
return "["+name+","+age+"]";
}

}

4.普通for循环遍历集合（size() 和 get(int index) 方法相结合）

import java.util.ArrayList;

//测试类
public class StudentTest {
public static void main(String[] args) {

System.out.println("使用普通for循环方法进行集合遍历的结果：");

// 创建学生类对象
Student s1 = new Student("国际米兰", 14);
Student s2 = new Student("罗马", 56);
Student s3 = new Student("贝尔格莱德", 45);

// 创建ArrayList集合对象
ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);

for (int i = 0; i < al.size(); i++) {
Student s = al.get(i)
4000
;
System.out.println(s);
}
}
}

// 学生类
class Student {
private String name;
private int age;

public Student() {
super();
}

public Student(String name, int age) {
super();
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;
}

@Override
public String toString() {
return "[" + name + "," + age + "]";
}

}

5.增强for循环进行集合的遍历

import java.util.ArrayList;

//测试类
public class StudentTest {
public static void main(String[] args) {

System.out.println("使用增强for循环方法进行集合遍历的结果：");

// 创建学生类对象
Student s1 = new Student("国际米兰", 14);
Student s2 = new Student("罗马", 56);
Student s3 = new Student("贝尔格莱德", 45);

// 创建ArrayList集合对象
ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);

//使用增强for循环行进遍历
for (Student s:al) {
System.out.println(s);
}
}
}

// 学生类
class Student {
private String name;
private int age;

public Student() {
super();
}

public Student(String name, int age) {
super();
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;
}

@Override
public String toString() {
return "[" + name + "," + age + "]";
}

}