Java
Search and filter
Write search and filtering with Stream.
Searching by title
As the blog grows, you need to find a specific post. A findByTitle(String keyword) method walks the posts list and collects posts whose title contains keyword into a new list.
Java's String.contains(keyword) checks for a substring. If title.contains("Java") returns true, we add that post to the result. The search is case-sensitive; for case-insensitive search use toLowerCase().
Collecting results in a new list
Inside the search method, you create an empty List<Post> result = new ArrayList<>();. The loop checks each post, and matching ones are added with result.add(post). After the loop, return result; gives back the list.
This pattern is the simplest way to filter without changing the original list. The Stream API can also do it, but we focus on the classic loop here.
Exercise Add the findByTitle method
Add a List<Post> findByTitle(String keyword) method to the Blog class. Inside, create a List<Post> result = new ArrayList<>();, loop with for (Post post : posts), check post.title.contains(keyword), add matches with result.add(post), and return result;. In main, add two posts (one with "Java" in the title), then print the size() of blog.findByTitle("Java").
Show example solution
class Post {
String title;
String author;
Post(String title, String author) {
this.title = title;
this.author = author;
}
}
class Comment {
String author;
String text;
Comment(String author, String text) {
this.author = author;
this.text = text;
}
}
class Blog {
List<Post> posts = new ArrayList<>();
void addPost(Post post) { posts.add(post); }
List<Post> findByTitle(String keyword) {
List<Post> result = new ArrayList<>();
for (Post post : posts) {
if (post.title.contains(keyword)) {
result.add(post);
}
}
return result;
}
}
public class Main {
public static void main(String[] args) {
Blog blog = new Blog();
blog.addPost(new Post("Java intro", "Ayşe"));
blog.addPost(new Post("Other", "Mehmet"));
System.out.println(blog.findByTitle("Java").size());
}
}
Test yourself
-
Complete the substring check
if (post.title.____(keyword)) {
result.add(post);
}
Show answer
if (post.title.contains(keyword)) {
result.add(post);
}
-
When does title.contains("Java") return true?
- When title equals Java exactly
- When title contains the Java substring
- When title starts with Java
- When title is the empty string
Show answer
When title contains the Java substring — contains checks for a substring; equality or prefix is not required.
-
Match the search concepts
- contains
- Substring check
- findByTitle
- Search by title method
- result
- List of matching posts
- toLowerCase
- Case-insensitive search
Next lesson: Putting it together
