Introduction to Java Collections and Strings

Introduction

Welcome to this course! Before we delve deeper into Java essentials for interview prep, let's start with some foundational Java features — specifically, ArrayLists and Strings . These features allow Java to group multiple elements, such as numbers or characters, under a single entity.

Understanding ArrayLists and Strings

Diving Into Lists

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Java

import java.util.Arrays;
import java.util.ArrayList;

class Solution {
    public static void main(String[] args) {
        // Defining an ArrayList and a String
        // <Integer> specifies the type of Objects the ArrayList will hold
        ArrayList<Integer> myList = new ArrayList<>(Arrays.asList(1, 2, 3, 4));
        String myString = "hello";

        // Now let's try to change the first element of both these features
        // set method changes the element at the specified index (0 in this case)
        myList.set(0, 100);
        // There is no method set for Strings
        // The following method does not change the string in place,
        // but returns a new string where 'h' is replaced with 'H'
        myString.replace('h', 'H');
        // So it is possible to obtain a new string like this:
        String newString = myString.replace('h', 'H');
        
        System.out.println(myList); // prints [100, 2, 3, 4]
        System.out.println(myString); // prints hello
        System.out.println(newString); // prints Hello
    }
}

Java

import java.util.ArrayList;
import java.util.Arrays;

class Solution {
    public static void main(String[] args) {
        // Creating an ArrayList
        ArrayList<String> fruits = new ArrayList<>(Arrays.asList("apple", "banana", "cherry"));

        // Add a new element at the end
        fruits.add("date"); // ['apple', 'banana', 'cherry', 'date']

        // Inserting an element at a specific position
        fruits.add(1, "bilberry"); // ['apple', 'bilberry', 'banana', 'cherry', 'date']

        // Removing a particular element
        fruits.remove("banana"); // ['apple', 'bilberry', 'cherry', 'date']

        // Accessing elements using indexing
        String firstFruit = fruits.get(0); // apple
        String lastFruit = fruits.get(fruits.size() - 1); // date

        // Converting static array to ArrayList and vice versa
        String[] fruitArray = {"kiwi", "lemon", "mango"};
        ArrayList<String> fruitList = new ArrayList<>(Arrays.asList(fruitArray));
        String[] newFruitArray = fruitList.toArray(new String[0]);
    }
}

Java

class Solution {
    public static void main(String[] args) {
        // Creating a string
        String greetingString = "Hello, world!";

        // Accessing characters using indexing
        char firstChar = greetingString.charAt(0);  // 'H'
        char lastChar = greetingString.charAt(greetingString.length() - 1);  // '!'

        // Making the entire string lowercase
        String lowercaseGreeting = greetingString.toLowerCase();  // "hello, world!"
    }
}

class Solution {
    public static void main(String[] args) {
        // Creating a string
        String greetingString = "Hello, world!";

        // Accessing characters using indexing
        char firstChar = greetingString.charAt(0);  // 'H'
        char lastChar = greetingString.charAt(greetingString.length() - 1);  // '!'

        // Making the entire string lowercase
        String lowercaseGreeting = greetingString.toLowerCase();  // "hello, world!"
    }
}

Java

class Solution {
    public static void main(String[] args) {
        // Using StringBuilder to create and modify a string
        StringBuilder sb = new StringBuilder("Hello");

        // Append characters to the string
        sb.append(", world!");

        // Insert characters at a specific position
        sb.insert(5, ",");

        // Convert StringBuilder back to a String
        String finalString = sb.toString();  // "Hello,, world!"
    }
}

class Solution {
    public static void main(String[] args) {
        // Using StringBuilder to create and modify a string
        StringBuilder sb = new StringBuilder("Hello");

        // Append characters to the string
        sb.append(", world!");

        // Insert characters at a specific position
        sb.insert(5, ",");

        // Convert StringBuilder back to a String
        String finalString = sb.toString();  // "Hello,, world!"
    }
}

Java

import java.util.ArrayList;
import java.util.Collections;
import java.util.Arrays;

class Solution {
    public static void main(String[] args) {
        // Define an ArrayList and a string
        ArrayList<Integer> myList = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
        String myString = "Hello";

        // Slicing: subList for ArrayList and substring for Strings
        ArrayList<Integer> sliceList = new ArrayList<>(myList.subList(2, 4)); // [3, 4]
        String sliceString = myString.substring(1, 3); // "el"

        // Concatenation: addAll for lists and + operator for strings
        ArrayList<Integer> concatenateList = new ArrayList<>(myList);
        concatenateList.addAll(Arrays.asList(6, 7, 8)); // [1, 2, 3, 4, 5, 6, 7, 8]
        String concatenateString = myString + ", world!"; // "Hello, world!"
        
        // Finding the index of an element in a list or a string
        // indexOf returns the first occurrence index of the element
        // returns -1 if the list or the string doesn't contain the element
        int indexList = myList.indexOf(3); // 2 - Index of element '3'
        int indexString = myString.indexOf('e'); // 1 - Index of character 'e'

        // Sorting items in ArrayList in non-increasing order
        ArrayList<Integer> sortedList = new ArrayList<>(myList);
        sortedList.sort(Collections.reverseOrder()); // [5, 4, 3, 2, 1]
    }
}

import java.util.ArrayList;
import java.util.Collections;
import java.util.Arrays;

class Solution {
    public static void main(String[] args) {
        // Define an ArrayList and a string
        ArrayList<Integer> myList = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
        String myString = "Hello";

        // Slicing: subList for ArrayList and substring for Strings
        ArrayList<Integer> sliceList = new ArrayList<>(myList.subList(2, 4)); // [3, 4]
        String sliceString = myString.substring(1, 3); // "el"

        // Concatenation: addAll for lists and + operator for strings
        ArrayList<Integer> concatenateList = new ArrayList<>(myList);
        concatenateList.addAll(Arrays.asList(6, 7, 8)); // [1, 2, 3, 4, 5, 6, 7, 8]
        String concatenateString = myString + ", world!"; // "Hello, world!"
        
        // Finding the index of an element in a list or a string
        // indexOf returns the first occurrence index of the element
        // returns -1 if the list or the string doesn't contain the element
        int indexList = myList.indexOf(3); // 2 - Index of element '3'
        int indexString = myString.indexOf('e'); // 1 - Index of character 'e'

        // Sorting items in ArrayList in non-increasing order
        ArrayList<Integer> sortedList = new ArrayList<>(myList);
        sortedList.sort(Collections.reverseOrder()); // [5, 4, 3, 2, 1]
    }
}

Java

class Solution {
    public static void main(String[] args) {
        // Creating a string
        String greetingString = "Hello, world!";

        // Accessing characters using indexing
        char firstChar = greetingString.charAt(0);  // 'H'
        char lastChar = greetingString.charAt(greetingString.length() - 1);  // '!'

        // Making the entire string lowercase
        String lowercaseGreeting = greetingString.toLowerCase();  // "hello, world!"
    }
}

Java

class Solution {
    public static void main(String[] args) {
        // Using StringBuilder to create and modify a string
        StringBuilder sb = new StringBuilder("Hello");

        // Append characters to the string
        sb.append(", world!");

        // Insert characters at a specific position
        sb.insert(5, ",");

        // Convert StringBuilder back to a String
        String finalString = sb.toString();  // "Hello,, world!"
    }
}

Java

import java.util.ArrayList;
import java.util.Collections;
import java.util.Arrays;

class Solution {
    public static void main(String[] args) {
        // Define an ArrayList and a string
        ArrayList<Integer> myList = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
        String myString = "Hello";

        // Slicing: subList for ArrayList and substring for Strings
        ArrayList<Integer> sliceList = new ArrayList<>(myList.subList(2, 4)); // [3, 4]
        String sliceString = myString.substring(1, 3); // "el"

        // Concatenation: addAll for lists and + operator for strings
        ArrayList<Integer> concatenateList = new ArrayList<>(myList);
        concatenateList.addAll(Arrays.asList(6, 7, 8)); // [1, 2, 3, 4, 5, 6, 7, 8]
        String concatenateString = myString + ", world!"; // "Hello, world!"
        
        // Finding the index of an element in a list or a string
        // indexOf returns the first occurrence index of the element
        // returns -1 if the list or the string doesn't contain the element
        int indexList = myList.indexOf(3); // 2 - Index of element '3'
        int indexString = myString.indexOf('e'); // 1 - Index of character 'e'

        // Sorting items in ArrayList in non-increasing order
        ArrayList<Integer> sortedList = new ArrayList<>(myList);
        sortedList.sort(Collections.reverseOrder()); // [5, 4, 3, 2, 1]
    }
}

Introduction

Understanding ArrayLists and Strings

Diving Into Lists

Join the 1M+ learners on CodeSignal

Understanding Strings

Indexing and Common Operations

Lesson Summary and Practice