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How do you check a list contains an item in Java?
Elements can be checked from a list using indexOf() or contains() methods.
Syntax - indexOf() method
int indexOf(Object o)
Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.
Parameters
o − Element to search for.
Returns
The index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
Throws
ClassCastException − If the type of the specified element is incompatible with this list (optional).
NullPointerException − If the specified element is null and this list does not permit null elements (optional).
Syntax - contains() method
boolean contains(Object o)
Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).
Parameters
o − Element whose presence in this list is to be tested.
Returns
True if this list contains the specified element.
Throws
ClassCastException − If the type of the specified element is incompatible with this list (optional).
NullPointerException − If the specified element is null and this list does not permit null elements (optional).
Example
Following is the example of finding elements from a list using various methods −
package com.tutorialspoint;
import java.util.ArrayList;
import java.util.List;
public class CollectionsDemo {
public static void main(String[] args) {
List<Student> list = new ArrayList<>();
list.add(new Student(1, "Zara"));
list.add(new Student(2, "Mahnaz"));
list.add(new Student(3, "Ayan"));
System.out.println("List: " + list);
Student student = new Student(3, "Ayan");
System.out.println("Ayan is present: " + list.contains(student));
int index = list.indexOf(student);
System.out.println("Ayan is present at: " + index);
}
}
class Student {
private int id;
private String name;
public Student(int id, String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public boolean equals(Object obj) {
if(!(obj instanceof Student)) {
return false;
}
Student student = (Student)obj;
return this.id == student.getId() && this.name.equals(student.getName());
}
@Override
public String toString() {
return "[" + this.id + "," + this.name + "]";
}
}Output
This will produce the following result −
List: [[1,Zara], [2,Mahnaz], [3,Ayan]] Ayan is present: true Ayan is present at: 2
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