java - 面向对象的Java

Question

我是一名学生，目前正在学习基本的面向对象java，我正在尝试创建一个OOP程序，它有3个类YouthGroup，部分和成员，一个Youthgroup有3个部分，这个程序的功能应该是不能列出成员在同一部分两次现在我正在尝试创建此功能:

public boolean setMembers(members members) {
    boolean crouded = true;

    for(int i = 0; i<this.getMembers().size();i++) {
        if(!this.members.contains(member)) {
            this.members.add(member);
        }
        else {
            System.out.println("Member already exists in this section");
            crouded = false;
        }
    }
    return crouded;
}

现在在主要方法中我正在执行以下操作:

public static void main(String[] args) {
    List<member> m = new ArrayList<>();
    Section s = new Section(m,"programming",3);
    member m1 = new member("tom",21,"male",s);
    member m2 = new member("kate",20,"Female",s);
    m.add(m1);
    m.add(m2);
    s.setMembers(m1);
    s.filterAge(21);
    System.out.println(s);
}

它给了我以下错误:

   Exception in thread "main" java.lang.StackOverflowError
    at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:648)
    at java.lang.StringBuilder.append(StringBuilder.java:208)
    at youthgroup.member.toString(member.java:62)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.member.toString(member.java:64)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at youthgroup.Section.toString(Section.java:68)
    at java.lang.String.valueOf(String.java:2981)
    at java.lang.StringBuilder.append(StringBuilder.java:131)

而且这种情况会永远持续下去。

这是成员类:

package youthgroup;

import java.util.Objects;

/**
 *
 * @author student
 */
public class member {
    private String name;
    private int age ; 
    private String gender;
    private Section section;

    public member(String name, int age, String gender, Section section) {
        this.name = name;
        this.age = age;
        this.gender = gender;
        this.section = section;
    }

    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 getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public Section getSection() {
        return section;
    }

    public void setSection(Section section) {
        this.section = section;
    }

    @Override
    public String toString() {
        String word = "Member name " +name;
        word+= "age : "+getAge();
        word+="Gender : "+ getGender();
        word+= " is in section : "+getSection();
        return word;
    }

    @Override
    public int hashCode() {
        int hash = 5;
        hash = 67 * hash + this.age;
        hash = 67 * hash + Objects.hashCode(this.gender);
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final member other = (member) obj;
        if (this.age != other.age) {
            return false;
        }
        if (!Objects.equals(this.gender, other.gender)) {
            return false;
        }
        return true;
    }





}

这是节类:

public class Section {
    private List<member>members;
    private String sectionName;
    private int maxNumber;

    public Section(List<member> members, String sectionName, int maxNumber) {
        this.members = members;
        this.sectionName = sectionName;
        this.maxNumber = maxNumber;
    }

    public List<member> getMembers() {
        return members;
    }

    public boolean setMembers(member member) {
        boolean crouded = true;

            for(int i = 0; i<this.getMembers().size();i++){
                if(!this.members.contains(member)){
                    this.members.add(member);
                }else{
                    System.out.println("Member already exists in this section");
                    crouded = false;
                }

            }



        return crouded;

    }

public String getSectionName() {
    return sectionName;
}

public void setSectionName(String sectionName) {
    this.sectionName = sectionName;
}

public int getMaxNumber() {
    return maxNumber;
}

public void setMaxNumber(int maxNumber) {
    this.maxNumber = maxNumber;
}

@Override
public String toString() {
    String word = " section name is : "+ getSectionName();
    word+=" Max number of members it can contain : "+ getMaxNumber();
    word+= " List of members :  "+getMembers();
    return word;
}

public boolean filterByAge(int a){
    boolean found = true;
    for(int i = 0; i<this.members.size(); i++){
        if(this.members.contains(a)){
            System.out.println("These members where found with this age  : "+ this.members.get(i).getName());
        }else{
            System.out.println("No member found !!!");
            found = false;
        }


    }

    return found;


}

我非常感谢您的帮助，提前谢谢您。

Answer 1

您的toString()方法正在对其自身进行递归调用。

toString()方法调用getSection()这将返回 Section将具有 toString() 的对象明确呼吁它。这个toString()本身会得到 List的member它将调用的对象 toString()然而，这一秒toString()方法本身会回调member toString()方法，它将一次又一次地回调。我想你已经看到问题了。

这将导致 StackOverflowError因为它将无限循环到调用堆栈中。

word+= " is in section : "+getSection();
//section is returned and called toString()
word+= " List of members :  "+getMembers();
//List of members are returned and called `toString()`, which will loop back to first line I've written

这里需要理解的重要一点是，当您使用+时会发生什么。 Java 中的运算符。当在 Objects 之间使用时，包括String物体或任何Objects您定义 +运算符将显式地将其左侧和右侧参数转换为 Strings 。在 Java 中完成此操作的方法是隐式调用 toString()您已重写的方法，但如果您没有重写它，它仍然存在(它位于 Object 父类(super class)中，因此所有 Java 对象都会继承一些基本实现)