【我的Java筆記】增強for迴圈 & ArrayList集合儲存自定義物件並遍歷的五種方法
阿新 • • 發佈：2018-12-25
增強for迴圈
1.增強for迴圈的作用：
在集合中，增強for迴圈的作用是用來代替迭代器
2.優點：書寫格式簡單，程式碼量少
3.弊端：遍歷陣列或集合的時候，陣列物件或者集合物件不能為null（需加上非空判斷）
注：在Java中，只要是物件，那麼一定要給物件做非空判斷，避免出現空指標異常
4.書寫格式：
for(陣列或者集合中的資料型別 變數名：陣列或者集合物件)｛
變數的輸出；
｝
例：
import java.util.ArrayList; public class Test2 { public static void main(String[] args) { //建立ArrayList集合物件 ArrayList<String> al = new ArrayList<String>(); //給集合中新增元素 al.add("hello"); al.add("world"); al.add("java"); //給集合物件進行非空判斷 if(al!=null) { //使用增強for迴圈遍歷集合 for(String str:al) { System.out.println(str); } } } }
ArrayList集合儲存自定義物件並遍歷的五種方法
1.使用集合遍歷（toArray()方法）：
import java.util.ArrayList; //測試類 public class StudentTest { public static void main(String[] args) { //建立學生類物件 Student s1 = new Student("國際米蘭",14); Student s2 = new Student("羅馬",56); Student s3 = new Student("貝爾格萊德",45); //建立ArrayList集合物件 ArrayList<Student> al = new ArrayList<Student>(); al.add(s1); al.add(s2); al.add(s3); //使用toArray()方法將集合轉為陣列 Object [] obj = al.toArray(); //遍歷陣列 for(int i=0;i<obj.length;i++) { //將物件obj[i]向下轉型 Student s = (Student)obj[i]; //輸出 System.out.println(s); } } } //學生類 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集合的迭代器進行集合遍歷（Iterator iterator()方法）
import java.util.ArrayList; import java.util.Iterator; //測試類 public class StudentTest { public static void main(String[] args) { System.out.println("使用Collection集合的迭代器Iterator iterator()方法進行集合遍歷的結果："); //建立學生類物件 Student s1 = new Student("國際米蘭",14); Student s2 = new Student("羅馬",56); Student s3 = new Student("貝爾格萊德",45); //建立ArrayList集合物件 ArrayList<Student> al = new ArrayList<Student>(); al.add(s1); al.add(s2); al.add(s3); //獲取迭代器物件 Iterator<Student> it = al.iterator(); while(it.hasNext()) { Student s = it.next(); System.out.println(s); } } } //學生類 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集合的迭代器進行遍歷（ListIterator listIterator()方法）
import java.util.ArrayList;
import java.util.ListIterator;
//測試類
public class StudentTest {
public static void main(String[] args) {
System.out.println("使用List集合的迭代器ListIterator listIterator()方法進行集合遍歷的結果：");
//建立學生類物件
Student s1 = new Student("國際米蘭",14);
Student s2 = new Student("羅馬",56);
Student s3 = new Student("貝爾格萊德",45);
//建立ArrayList集合物件
ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);
//獲取迭代器物件
ListIterator<Student> it = al.listIterator();
while(it.hasNext()) {
Student s = it.next();
System.out.println(s);
}
}
}
//學生類
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迴圈遍歷集合（size() 和 get(int index) 方法相結合）
import java.util.ArrayList;
//測試類
public class StudentTest {
public static void main(String[] args) {
System.out.println("使用普通for迴圈方法進行集合遍歷的結果：");
// 建立學生類物件
Student s1 = new Student("國際米蘭", 14);
Student s2 = new Student("羅馬", 56);
Student s3 = new Student("貝爾格萊德", 45);
// 建立ArrayList集合物件
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);
System.out.println(s);
}
}
}
// 學生類
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.增強for迴圈進行集合的遍歷
import java.util.ArrayList;
//測試類
public class StudentTest {
public static void main(String[] args) {
System.out.println("使用增強for迴圈方法進行集合遍歷的結果：");
// 建立學生類物件
Student s1 = new Student("國際米蘭", 14);
Student s2 = new Student("羅馬", 56);
Student s3 = new Student("貝爾格萊德", 45);
// 建立ArrayList集合物件
ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);
//使用增強for迴圈行進遍歷
for (Student s:al) {
System.out.println(s);
}
}
}
// 學生類
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 + "]";
}
}