The wish to write a technology blog every week has never come true , Starting this week, every Friday night 10 It's time to blog

OOP One of the most important characteristics of the Internet is polymorphism , There are many ways to achieve polymorphism . example : heavy load overload、 rewrite overwite、 Interface oriented programming and so on . However, overloading should be used with caution in practical application , This is also Effective Java One of the things mentioned in . Let's show it first eJava The code mentioned in :


public void testOverWrite(){

List<Integer> intList = new ArrayList<Integer>();

Set<Integer> intSet = new HashSet<Integer>();

for(int i = -3 ; i < 3 ; i++){




System.out.println(intList+" ---> "+intSet);

for(int i =0 ; i< 3 ;i++){




System.out.println(intList+" ### "+intSet);


Suppose there is no test Many people may think that the output is like this :

[-3, -2, -1, 0, 1, 2]  --->  [0, 1, 2, -3, -2, -1]

[-3, -2, -1] ###  [-3, -2, -1]

But it turns out that :

[-3, -2, -1, 0, 1, 2]  --->  [0, 1, 2, -3, -2, -1]

[-2, 0, 2]  ###  [-3, -2, -1]

The first line must be OK ,intSet Also no problem .intList Maybe a lot of people will have questions ‘’ Why with intSet It's different. ‘

In fact JDK5 I didn't have that problem before ,jdk5 And later added their own active packaging box function , The basic type and the reference type will help you to convert .

This leads to List stay remove It's the index that's removed when it's done , Instead of the data in the container you think .

public E remove(int index) {



        E oldValue = elementData(index);

        int numMoved = size - index - 1;

        if (numMoved > 0)

            System.arraycopy(elementData, index+1, elementData, index,


        elementData[--size] = null; // Let gc do its work

        return oldValue;


Instead of this function :

public boolean remove(Object o) {

        if (o == null) {

            for (int index = 0; index < size; index++)

                if (elementData[index] == null) {


                    return true;


        } else {

            for (int index = 0; index < size; index++)

                if (o.equals(elementData[index])) {


                    return true;



        return false;


jdk I took the initiative to help you unpack . and HashSet No, remove The index method is called remove object

public boolean remove(Object o) {

        return map.remove(o)==PRESENT;


So there won't be list That kind of question .

So when List remove One Integer You need to pay attention to , It's very likely that it doesn't turn out to be the function you want .

------------------------- Beautiful cutting line ——————————————————

Two 、 When the parameter list is similar , It's better not to use overloading . In particular, exporting public API. most easy yes Users Cause confusion . Like the one I met today Money Class has two functions with the same parameter list :multiply and multiplyBy, Have the same parameter list . The first time I used it, I followed in and opened the code , Keep sth. in mind multiply Inside is the new new I got an object , The value of the original object remains unchanged . Also understand that this value cannot be changed . But the line code was optimized before the launch , Used ’multiply‘. Only the first half of the test was followed up , It turns out that the data is right . It turns out that at the end of the day there's another problem , It turns out that multiplyBy, And the function changes the original object . A waste of time . Why not write the full name ? A function name can probably use at most 65535 Character length , It seems that no matter how complex the business function name is, it can't be so long .

———————— Gorgeous cutting line ————————————————-———

3、 ... and 、 Observation Code :

private static void printClassName(Set<?> set){



private static void printClassName(List<?> list){



private static void printClassName(Collection<?> col){

System.out.println("unknow class name...");


public static void main(String[] args) {

String[] str = {"a","b"};

Collection<Integer>[] cols = {

new HashSet<Integer>(),

new ArrayList<Integer>(),

new HashMap<Integer,Integer>()


for(Collection col : cols){




overwiter Is implemented between parent and child classes ,overload Is implemented in the same class . therefore overload It's the compile time . Which method to call depends on the type of reference . So the above three times will print ’unknow class name‘. Due to compiler col All are collection Type of .

and overload The method is selected according to the type of the instance of the class in which the method is called , So we will use the duplicated implementation in the subclass .

Effective_java The second : Use overloaded functions with caution

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