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Half of the people don't know the difference between for and foreach???

2022-07-07 11:30:00 【Java technology stack】

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A colleague suddenly asked me a question , Said in foreach Can I delete it list The elements inside , I said about whether I could delete , And why ; Next, let's explore whether this can be done ;

（1） Traversing elements

First , Let's take a piece of code as an example ：

String[] array = {"1", "2", "3"};
for (String i : array) {
    System.out.println(i);
}

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
for (String i : list) {
    System.out.println(i);
}

After traversal, the results are as follows ：

There is no doubt that .

Let's take a look at the compiled source code （idea Bring their own , stay target Open your class source file in the package ）：

String[] array = new String[]{"1", "2", "3"};
String[] var2 = array;
int var3 = array.length;

for(int var4 = 0; var4 < var3; ++var4) {
    String i = var2[var4];
    System.out.println(i);
}

ArrayList<String> list = new ArrayList();
list.add("111");
list.add("222");
list.add("333");
Iterator var7 = list.iterator();

while(var7.hasNext()) {
    String i = (String)var7.next();
    System.out.println(i);
}

so , Traversing the array uses the original for loop , The collection uses Iterator iterator . The latest interview questions have been sorted out , You can Java Interview library applet online brush questions .

（2） Remove elements

Oh, yes k！ Next, let's delete the element ：

Use for loop ：

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
for (int i = 0; i <list.size(); i++) {
    list.remove("222");
}

log.info(list.toString());

result ：

11:11:52.532 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 222, 333]
11:11:52.539 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 333]

Obviously successful ！

Use foreach：

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
for (String i : list) {
  list.remove("222");
}
log.info(list.toString());

result ：

11:50:48.333 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 222, 333]
Exception in thread "main" java.util.ConcurrentModificationException
 at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
 at java.util.ArrayList$Itr.next(ArrayList.java:859)
 at com.xiaolinge.com.hello.HelloWord.main(HelloWord.java:30)

Obviously, wood has success ！

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reason ：

Every traversal inside the iterator will record List Inside modcount As expected , Then use the expected value and in each cycle List Member variables of modCount The comparison , But ordinary list.remove It's called List Of remove, At this time modcount++, however iterator The expected value recorded in = There is no change , So there's an error .

If you want to delete an element, you need to use the inside of the iterator remove Method ：

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
Iterator<String> it = list.iterator();
while (it.hasNext()){
    String next = it.next();
    //if For external use list Of remove The method will still report an error 
    if(next.equals("222")){
        it.remove();// Here we use the inside of the iterator remove（） Method ,
        //  Of course if you use list Of remove If the method deletes texture elements here, it is successful , such as ：list.remove("222")
    }
}
log.info(list.toString());

result ：

12:06:14.042 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 222, 333]
12:06:14.046 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 333]

（3） Modifying elements

Use primitive for：

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
for (int i = 0; i <list.size(); i++) {
    list.set(i,"444");
}
 log.info(list.toString());

result ：

12:12:56.910 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 222, 333]
12:12:56.915 [main] INFO com.xiaolinge.com.hello.HelloWord - [444, 444, 444]

Oh, yes k！ You can modify elements ;

Use foreach：

ArrayList<String> list = new ArrayList<>();
 list.add("111");
 list.add("222");
 list.add("333");
 log.info(list.toString());
for (String i : list) {
     i="444";
 }
  log.info(list.toString());

result ：

12:34:47.207 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 222, 333]
12:34:47.211 [main] INFO com.xiaolinge.com.hello.HelloWord - [111, 222, 333]

See , No way .

Spicy? , You can't modify elements , Can you modify the attributes of an element ？ Let's take a look . The latest interview questions have been sorted out , You can Java Interview library applet online brush questions .

（4）foreach Modify element properties

（for I won't test it ）

Create a student class ：

public class Student {
        private int age;
        public int getAge() {
            return age;
        }
        public void setAge(int age) {
            this.age = age;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        private String name;
        public Student(){};
        public Student(int age,String name){
            this.age=age;
            this.name=name;
        }
    }

Oh, yes k, Next, test the code ：

Student student=new Student(1,"huge");
        Student student1=new Student(1,"xiaoyao");
        List<Student> studentList=new ArrayList<Student>();
        studentList.add(student);
        studentList.add(student1);
        System.out.println(student.getName());
        System.out.println(student1.getName());
        for(Student stu:studentList)
        {
            stu.setName("jingtian");
        }
        System.out.println(student.getName());
        System.out.println(student1.getName());

result ：

huge
xiaoyao
jingtian
jingtian

484 Amazing ！ Can't modify the object , But you can modify the properties of the object .

summary

for And foreach Can traverse arrays / aggregate , however for It is more efficient in more complex cycles .
foreach Cannot delete / Modify set elements , and for Sure
foreach and for You can modify the attributes in the element

So by comparison for More flexible loops .

Copyright notice ： This paper is about CSDN Blogger 「coder Brother Xiao Lin 」 The original article of , follow CC 4.0 BY-SA Copyright agreement , For reprint, please attach the original source link and this statement . Link to the original text ：https://blog.csdn.net/qq_40521656/article/details/90749927