In the tutorial, we will show how to find an element in a List Object with Java APIs and Google Guava library.
contains()
Signature method:
boolean java.util.List.contains(Object o) |
-> Returns true if the list contains at least one element e such that Objects.equals(o, e).
Example:
List<Integer> integerList = Arrays.asList(1, 3, 7, 9, 11); System.out.println(integerList.contains(3)); // true System.out.println(integerList.contains(4)); // false |
Example with Custom Object
– Define a Customer class:
class Customer{ private int id; private String name; Customer(int id, String name){ this.id = id; this.name = name; } public int getId() { return id; } public String getName() { return name; } ... } |
– We also need define equals() method for Customer class as below to compare Objects:
public boolean equals(Object o) { // If the object is compared with itself then return true if (o == this) { return true; } /* Check if o is an instance of Customer or not "null instanceof [type]" also returns false */ if (!(o instanceof Customer)) { return false; } // typecast o to Customer so that we can compare data members Customer c = (Customer) o; // Compare the data members and return accordingly if(id == c.id && name.equals(c.name)) return true; else return false; } |
-> Now we do an example using contains() method with custom object list as below:
List<Customer> customers = Arrays.asList(new Customer(1, "Jack"), new Customer(2, "Mary"), new Customer(3, "Harry"), new Customer(4, "Joe"), new Customer(5, "Jane")); System.out.println(customers.contains(new Customer(2, "Mary"))); // true System.out.println(customers.contains(new Customer(3, "Jane"))); // false |
indexOf()
Signature:
int java.util.List.indexOf(Object o) |
– indexOf returns the lowest index i such that Objects.equals(o, get(i)), or -1 if there is no such index.
Example:
List<Customer> customers = Arrays.asList(new Customer(1, "Jack"), new Customer(2, "Mary"), new Customer(3, "Harry"), new Customer(4, "Joe"), new Customer(5, "Jane")); int index = customers.indexOf(new Customer(3, "Harry")); System.out.println(index); // 2 index = customers.indexOf(new Customer(7, "Mary")); System.out.println(index); // -1 |
Looping to Find Element in Java List
– Example 1:
public static void main(String[] args) { List<Customer> customers = Arrays.asList(new Customer(1, "Jack"), new Customer(2, "Mary"), new Customer(3, "Harry"), new Customer(4, "Joe"), new Customer(5, "Jane")); Customer cust = findCustomer("Joe", customers); System.out.println("Customer id=" + cust.getId() + ", name=" + cust.getName()); //Customer id=4, name=Joe } public static Customer findCustomer(String name, List<Customer> customers) { for(Customer c: customers) { if(name.equals(c.getName())) { return c; } } return null; } |
– Example 2 with Iterator implementation:
public static Customer findCustomer(String name, List<Customer> customers) { Iterator<Customer> iterator = customers.iterator(); while(iterator.hasNext()) { Customer c = iterator.next(); if(c.getName().equals(name)) { return c; } } return null; } |
Use Stream Filter of Java 8
We use Predicate to filter an element with Java 8 Stream:
java.util.stream.Stream.filter(Predicate<? super Customer> predicate) |
And combine with findFirst method. Detail Example:
List<Customer> customers = Arrays.asList(new Customer(1, "Jack"), new Customer(2, "Mary"), new Customer(3, "Harry"), new Customer(4, "Joe"), new Customer(5, "Jane")); Customer cust = customers.stream().filter(c -> c.getName() == "Joe").findFirst().orElse(null); System.out.println("Customer id = " + cust.getId() + ", name = " + cust.getName()); // Customer id = 4, name = Joe |
Use Google Guava
– Dependency:
<!-- https://mvnrepository.com/artifact/com.google.guava/guava --> <dependency> <groupId>com.google.guava</groupId> <artifactId>guava</artifactId> <version>27.1-jre</version> </dependency> |
– Example:
Customer cust = Iterables.find(customers, new Predicate<Customer>() { public boolean apply(Customer customer) { return "Jane".equals(customer.getName()); } }, null); |
Full code:
public static void main(String[] args) { List<Customer> customers = Arrays.asList(new Customer(1, "Jack"), new Customer(2, "Mary"), new Customer(3, "Harry"), new Customer(4, "Joe"), new Customer(5, "Jane")); Customer c = Iterables.find(customers, new Predicate<Customer>() { public boolean apply(Customer customer) { return "Jane".equals(customer.getName()); } }, null); if(null!=c){ System.out.println("Customer id = " + cust.getId() + ", name = " + cust.getName()); } // Customer id = 5, name = Jane } |
Conclusion
We had learned the way to find an element in Java Object List with Examples by using:
- contains()
- indexOf()
- Looping to find Element
- Use Stream Filter of Java 8
- Google Guava
Happy learning! See you later!