How do I know if an array list of objects contain a certain string?

Question

I made a method that search through an array list of objects. Then if the searchKey is found in the array list it will print this certain item.

Here is how I iterate through the array list if it contains the searchKey, but I just realized that it is impossible to compare a string and an object.

for(int x = 0; x < Student.students.size(); x ++){
 if(Student.students.contains(searchKey))
 System.out.print(Student.students.get(x));
}

Here's how I create the constructor and array list.

String firstName, lastName, course, yearLevel, gender;
 
 Student(String firstName, String lastName, String course, String yearLevel, String gender)
 {
 this.firstName = firstName;
 this.lastName = lastName;
 this.course = course;
 this.yearLevel = yearLevel;
 this.gender = gender;
 }
 static ArrayList<Student> students = new ArrayList<Student>();

• azro: please share the List instanciation and class of the object into it
• user16320675: what are you searching for? first name, last name, course, ... or gender? any of these? exact or partial match?

Answer 1

You need to compare the one property; also you can use a for-each loop to simplify the code

for(Student s : Student.students){
 if(s.getName().equals(searchKey))
 System.out.print(s);
}

---

Note :

When you use a condition Student.students.contains(searchKey) in a loop, and it doesn't use the iteration variable that means there is a problem

• Silly Bean: I posted how I create the list. I also try the method you just mentioned, but it wont work as it is still comparing a string to an object
• Jon Skeet: @SillyBean: No, it's comparing the name of the student to the search key...
• daniu: You can also use a stream, students.stream().filter(s -> searchKey.equals(s.getName()).forEach(System.out::println).
• azro: @SillyBean what is searchKey a string or a Student ?

Answer 2

You haven't defined what 'contains' means here but I'm going to assume that it means a Student contains the key if it appears as a substring in any of its String members. So let's start with a method that does that. We can define this as part of the Student class itself.

public class Student {
 .... other stuff ....
 /**
 * Return true if any of the properties of this Student
 * contain the given substring, false otherwise
 */
 public boolean contains(String s) {
 // consider addressing null cases - omitting for simplicity
 return firstName.contains(s) ||
 lastName.contains(s) ||
 course.contains(s) ||
 yearLevel.contains(s) ||
 gender.contains(s);
 }
}

Now you can iterate over your List and invoke this method to find the matches. Note that you need to handle the case that multiple Students may match a given search key (or none may match). So I would suggest collecting the results in a separate List. One does not generally iterate over Lists via the index. This example uses an enhanced for-loop (aka for-each).

public List<Student> findMatches(List<Student> students, String key) {
 List<Student> found = new ArrayList<>();
 for (Student s : students) {
 if (s.contains(key)) {
 found.add(s);
 }
 }
 return found;
}

This is a good case for using the Stream API.

public List<Student> findMatches(List<Student> students, String key) {
 return students.stream()
 .filter(s -> s.contains(key))
 .collect(Collectors.toList());
}