How to Convert List to Map in Java 8

If you have a scenario where you need to convert List of objects to a Map, prior to java 8 you can do this by writing a simple for loop which is iterative. But in this article I will show you how to convert List to Map using java 8 lambda expression.

Java 8 Convert List to Map Example

Lets a take pojo class named Account with properties accNo, accType, accStatus.

Account.java

12345678910111213141516171819202122232425262728293031public class Account {
	
	private int accNo;
	private String accType;
	private String accStatus;
	
	public Account(int accountNo, String accountType, String accountStatus){
		accNo = accountNo;
		accType = accountType;
		accStatus = accountStatus;
	}
	
	public int getAccNo() {
		return accNo;
	}
	public void setAccNo(int accNo) {
		this.accNo = accNo;
	}
	public String getAccType() {
		return accType;
	}
	public void setAccType(String accType) {
		this.accType = accType;
	}
	public String getAccStatus() {
		return accStatus;
	}
	public void setAccStatus(String accStatus) {
		this.accStatus = accStatus;
	}		
}

ListtoMap.java

1234567891011121314151617181920212223242526272829303132import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ListtoMap {
	
	public static void main(String[] args) {

		List<Account> li = new ArrayList<Account>();
		
		Account ac1= new Account(100,"Savings","Open");
		Account ac2= new Account(101,"Checking","Closed");
		
		li.add(ac1);
		li.add(ac2);
		
	 // Before Java 8
		Map<Integer, String> beforeJava8 = new HashMap<>(); 
		for (Account acc: li) {
			beforeJava8.put(acc.getAccNo(), acc.getAccStatus()); 
		} 
	    System.out.println("Before Java 8 "+beforeJava8);		    
	    
	    
	    
	 // Java 8 Using Lambda expression and Stream API
		Map<Integer, String> map = li.stream().collect(Collectors.toMap(x -> x.getAccNo()  , x -> x.getAccStatus()));
		System.out.println("Java 8 "+map);
	}
}

Output

Before Java 8 {100=Open, 101=Closed}
Java 8  {100=Open, 101=Closed}

Hey its same output 😉 I just tried to show both ways [ with and without lambda expression ].

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::. About the Author .::
		Sivateja Kandula - Java/J2EE Full Stack Developer Founder of Java4s - Get It Yourself, A popular Java/J2EE Programming Blog, Love Java and UI frameworks. You can sign-up for the Email Newsletter for your daily dose of Java tutorials.