How to create a Java HashMap of user defined class type? - GeeksforGeeks

04 Mar, 2019
Pre-requisite: Internal working of HashMap, HashMapIf we wish to create a HashMap of our own class, we need to ensure that the hashcode() of the key of HashMap doesn’t change as if it happens then it is impossible to get object value of the key from HashMap.
On runtime, JVM processes hash code for each object and give it on interest. When we alter an objects’ state, JVM calculates its hash code again which may result in memory leak. To make things work what we have to do is make sure that state change for a key object does not change the hash code of object i.e. the key must have properly overridden equals() and hashcode() methods for it to work correctly.
One of the ways of doing this is by making key objects IMMUTABLE. Immutability allows you to get same hashcode every time, for a key object. This is the primary motivation behind why Immutable classes like String, Integer or other wrapper classes are a decent key object applicant. Learn more about this here- How to make class Immutable?
// Java example to create a Java HashMap// of user-defined class type  import java.util.*;import java.io.*;  // User defined classpublic class CustomKeyObject {      public static class Student {          private int rollno;        private String name;          // Constructor        public Student(int rollno, String name)        {            this.rollno = rollno;            this.name = name;        }          public String getname()        {            return this.name;        }          public int getmarks()        {            return this.rollno;        }          public void setname(String name)        {            this.name = name;        }          public void setmarks(int rollno)        {            this.rollno = rollno;        }          // Overriding the hashcode() function        @Override        public int hashCode()        {              // uses roll no to verify the uniqueness            // of the object of Student class              final int temp = 14;            int ans = 1;            ans = temp * ans + rollno;            return ans;        }          // Equal objects must produce the same        // hash code as long as they are equal        @Override        public boolean equals(Object o)        {            if (this == o) {                return true;            }            if (o == null) {                return false;            }            if (this.getClass() != o.getClass()) {                return false;            }            Student other = (Student)o;            if (this.rollno != other.rollno) {                return false;            }            return true;        }    }      // main method    public static void main(String[] args)        throws NumberFormatException,               IOException    {        HashMap<Student, String> map = new HashMap<>();          Student one = new Student(1, "Geeks1"); // key1          Student two = new Student(2, "Geeks2"); // key2          // put keys in map        map.put(one, one.getname());        map.put(two, two.getname());          // changing key state so that        // hashcode() should be calculated again        one.setname("Not Geeks1");        two.setname("Not Geeks2");          // still prints Geeks1        System.out.println(map.get(one));          // still prints Geeks1        System.out.println(map.get(two));          // try with newly created key with same Rollno        Student three = new Student(1, "Geeks3");          // we get Geeks1        System.out.println(map.get(three));    }      // This code is contributed by Subhesh}
Geeks1
Geeks2
Geeks1

Java-HashMap
Picked
Java
Java
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Stream In Java
Constructors in Java
Exceptions in Java
Functional Interfaces in Java
Different ways of Reading a text file in Java
Generics in Java
Introduction to Java
Comparator Interface in Java with Examples
Internal Working of HashMap in Java
Strings in Java