面向对象的Java

Question

我是一名正在学习基本面向对象java的学生，我正在尝试创建一个OOP程序，其中有3个类YouthGroup，Section和成员一个Youthgroup有3个部分，这个程序的功能应该是一个成员不能列出在同一部分两次，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，我正在做以下事情：

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)

它会永远持续下去。

这是会员类：

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;
    }





}

这是部分类：

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;


}

我真的很感激任何帮助，谢谢你。

Answer 1

您的toString()方法正在对自己进行递归调用。

toString()方法调用getSection()将返回Section对象，该对象将toString()显式调用它。这个toString()本身会获得List member个toString()个对象，它会调用toString()，但是，第二个member方法本身会回调toString() {1}} StackOverflowError方法，将再次回拨，再次回拨。我想，你看到了问题。

这将导致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 ，因为它将无限循环到调用堆栈中。

+

这里要理解的重要一点是在Java中使用Objects运算符时会发生什么。在String，包括Objects个对象或您定义的任何+之间使用时，Strings运算符会将其左侧和右侧参数显式转换为toString() 。在Java中完成此操作的方式是对您已覆盖的Object方法的隐式调用，但如果您没有覆盖它，它仍然存在（它位于{{1}}超类中，所以全部Java对象继承了一些基础实现）