object-references

Object References Exercises

This directory contains exercises to help you practice working with object references in Java using jshell.

Setup

1. Open your terminal/command prompt
2. Navigate to this directory
3. Start jshell: jshell
4. Load the exercise file: /open topic.jsh

Exercises

Exercise 1: Reference Basics

Write a method createStringArray() that creates and returns an array of 3 Strings. Write a method areReferencesEqual(String str1, String str2) that checks if two String references point to the same object (using ==). Write a method areContentsEqual(String str1, String str2) that checks if two String references have the same content (using equals).

Exercise 2: Null Handling

Write a method isNullReference(Object obj) that returns true if the reference is null. Write a method safeToString(Object obj) that converts an object to string, returning "null" if the object is null. Write a method safeLength(String str) that returns the length of a string, or 0 if null.

Exercise 3: Array References

Write a method copyArrayReference(int[] original) that copies the reference (not the content) of an array. Write a method copyArrayContent(int[] original) that creates a new array with the same content. Write a method modifyArray(int[] array, int index, int newValue) that changes a value in the array.

Exercise 4: Object State Changes

Write a method createStringBuilder(String initial) that creates a StringBuilder with initial text. Write a method appendToBuilder(StringBuilder sb, String text) that adds text to the StringBuilder. Write a method getBuilderContent(StringBuilder sb) that returns the current content as a String.

Exercise 5: Reference Comparison

Write a method findStringInArray(String[] array, String target) that finds the first String that has the same content as target. Write a method countNullReferences(Object[] array) that counts how many elements in the array are null. Write a method replaceNulls(String[] array, String replacement) that replaces all null elements with the replacement string.

Exercise 6: Multiple References

Write a method demonstrateStringPool() that creates two String literals with the same value and shows they reference the same object. Write a method demonstrateNewString() that creates two String objects with new keyword and shows they are different objects. Write a method swapReferences(StringBuilder[] array, int index1, int index2) that swaps two references in an array.

Exercise 7: Object Creation and References

Write a method createArrayList() that creates and returns a new ArrayList. Write a method addToList(java.util.ArrayList<String> list, String item) that adds an item to the list. Write a method getListReference(java.util.ArrayList<String> list) that returns the same list reference.

Testing Your Code

After completing each exercise, test your methods with different inputs:

// Example tests
String[] array = createStringArray();
String str1 = "Hello";
String str2 = "Hello";
String str3 = new String("Hello");

areReferencesEqual(str1, str2)      // Should return true (string pool)
areReferencesEqual(str1, str3)      // Should return false (different objects)
areContentsEqual(str1, str3)        // Should return true (same content)

isNullReference(null)               // Should return true
safeLength(null)                    // Should return 0
safeLength("Hello")                 // Should return 5

Tips

• Read the NOTES.md file first to understand object references
• Remember the difference between == (reference equality) and .equals() (content equality)
• Always check for null before calling methods on objects
• Understand that arrays and objects are passed by reference
• Objects created with new are always different from each other
• String literals may reference the same object due to string pooling