Mahmoud Abas, Full-Stack Software Engineer & AI

Software Construction - Software Construction - Lab 2: Java Collections, Arrays, Loops & Classes

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Software Construction - Lab 2

Java Collections, Arrays, Loops & Classes

From loops and arrays to OOP, references, and the Collections Framework.

Lab Objectives

Primitive vs object values and how Java manages memory
Loops (for, while) and iteration patterns
Arrays and array indexing
Classes, Objects, Constructors, Methods (OOP)
References vs Values, the static and final keywords
Mutating values vs reassigning variables
Java Collections Framework: List, Set, Map

Good Programming Style

Use meaningful variable names (firstName, not a1)
Use proper indentation - Eclipse: Ctrl+Shift+F to auto-format
Use whitespace for readability around operators
Don't duplicate logic - a plain else is enough once prior conditions rule out the rest
Always use curly braces, even for single-statement loops/ifs - omitting them is a classic bug source

Loops and Iteration

java

// while loop
int i = 0;
while (i < 3) {
    System.out.println("Count: " + i);
    i = i + 1;  // must change i or it loops forever!
}

// for loop
for (int i = 0; i < 3; i++) {
    System.out.println("Count: " + i);
}

break exits the loop immediately
continue skips to the next iteration

Arrays

Array: An indexed list of values of the same type.

Index 0 is the first element; values[i] is a value, i is just a number - don't confuse index with value

java

int[] numbers = {12, 24, -23, 47, 100};
int size = 10;
double[] data = new double[size];

Worked Example

Array Algorithm: Find Minimum Index

java

public static int getMinIndex(int[] values) {
    int minValue = Integer.MAX_VALUE;
    int minIndex = -1;
    for (int i = 0; i < values.length; i++) {
        if (values[i] < minValue) {
            minValue = values[i];
            minIndex = i;
        }
    }
    return minIndex;
}

Same pattern extends to second minimum by skipping the first minimum's index

Debugging Techniques

Add strategic print statements to trace state at each loop iteration
Print before and after key decisions (for example, "New minimum found!")
Keep code formatted (Ctrl+Shift+F in Eclipse) - messy code hides bugs

Classes and Objects

Managing many related values with separate primitive variables doesn't scale - group them into a class

java

public class Student {
    String name;
    int age;
    double gpa;

    Student(String studentName, int studentAge) {
        name = studentName;
        age = studentAge;
        gpa = 0.0;
    }

    void displayInfo() {
        System.out.println(name + " (" + age + "): GPA " + gpa);
    }
}

Fields = data, constructor = initialization, methods = behavior

Creating and Using Objects

java

Student ahmed = new Student("Ahmed Ali", 20);
Student fatima = new Student("Fatima Hassan", 19);

System.out.println(ahmed.name);   // "Ahmed Ali"
ahmed.displayInfo();

new allocates the object; fields are accessed with .; methods are called with .

References vs Values

Primitives: Copied on assignment - y = x gives y its own copy.

Objects: Stored as references - s2 = s1 makes s2 point to the same object.

java

Student s1 = new Student("Ahmed", 20, "CS");
Student s2 = s1;        // s2 points to SAME object
s1.gpa = 3.8;
System.out.println(s2.gpa);  // 3.8 (same object!)

Student s3 = new Student("Ahmed", 20, "CS");
System.out.println(s1 == s3);  // false ,  different objects

The `static` Keyword

java

public class Student {
    static int totalStudents = 0;  // SHARED by all instances
    String name;                    // UNIQUE to each instance

    Student(String studentName) {
        name = studentName;
        totalStudents++;
    }
}

Static methods can be called without creating an object (for example, MathHelper.add(5, 3))
main is static because Java must call it before any objects exist

Mutation vs Reassignment, and `final`

Reassignment - x = 10; changes what the variable points to
Mutation - ahmed.gpa = 3.8; changes the object's internal state; ahmed still points to the same object

java

final int[] scores = {85, 90, 78};
scores[0] = 95;              // OK: mutating contents
// scores = new int[]{1,2,3}; // ERROR: reassignment not allowed

final locks a reference, not the object's contents

Java Collections: List, Set, Map

java

List<String> courses = new ArrayList<>();
courses.add("Math");
courses.add("Physics");

Set<Integer> ids = new HashSet<>();
ids.add(101);
ids.add(101);              // duplicate ,  ignored
System.out.println(ids.size());  // 2, not 3

Map<String, Double> grades = new HashMap<>();
grades.put("Ahmed", 3.8);
System.out.println(grades.get("Ahmed"));  // 3.8

List - ordered, allows duplicates
Set - no duplicates
Map - key/value pairs (for example, zip codes to cities, managers to employee lists)

Key Takeaways

Loops (for/while) with break/continue drive iteration; arrays hold fixed-length, same-type sequences
Classes bundle fields (data), constructors (initialization), and methods (behavior) into objects
Primitives copy on assignment; objects are shared by reference - == compares references, not contents
static members belong to the class and are shared across all instances; final locks a reference, not the value it points to
Mutating an object changes its state; reassigning a variable changes what it points to - these are different operations
Choose the right collection: List for ordered/duplicate data, Set for uniqueness, Map for key-to-value lookups

Open this lesson

Mahmoud AbasSoftware Construction - Lab 2: Java Collections, Arrays, Loops & Classes

// while loop int i = 0; while (i < 3) { System.out.println("Count: " + i); i = i + 1; // must change i or it loops forever! } // for loop for (int i = 0; i < 3; i++) { System.out.println("Count: " + i); }

public static int getMinIndex(int[] values) { int minValue = Integer.MAX_VALUE; int minIndex = -1; for (int i = 0; i < values.length; i++) { if (values[i] < minValue) { minValue = values[i]; minIndex = i; } } return minIndex; }

public class Student { String name; int age; double gpa; Student(String studentName, int studentAge) { name = studentName; age = studentAge; gpa = 0.0; } void displayInfo() { System.out.println(name + " (" + age + "): GPA " + gpa); } }

Student s1 = new Student("Ahmed", 20, "CS"); Student s2 = s1; // s2 points to SAME object s1.gpa = 3.8; System.out.println(s2.gpa); // 3.8 (same object!) Student s3 = new Student("Ahmed", 20, "CS"); System.out.println(s1 == s3); // false , different objects

public class Student { static int totalStudents = 0; // SHARED by all instances String name; // UNIQUE to each instance Student(String studentName) { name = studentName; totalStudents++; } }

List<String> courses = new ArrayList<>(); courses.add("Math"); courses.add("Physics"); Set<Integer> ids = new HashSet<>(); ids.add(101); ids.add(101); // duplicate , ignored System.out.println(ids.size()); // 2, not 3 Map<String, Double> grades = new HashMap<>(); grades.put("Ahmed", 3.8); System.out.println(grades.get("Ahmed")); // 3.8

Java Collections, Arrays, Loops & Classes

Lab Objectives

Good Programming Style

Loops and Iteration

Arrays

Array Algorithm: Find Minimum Index

Debugging Techniques

Classes and Objects

Creating and Using Objects

References vs Values

The static Keyword

Mutation vs Reassignment, and final

Java Collections: List, Set, Map

Key Takeaways

Open this lesson

Java Collections, Arrays, Loops & Classes

Lab Objectives

Good Programming Style

Loops and Iteration

Arrays

Array Algorithm: Find Minimum Index

Debugging Techniques

Classes and Objects

Creating and Using Objects

References vs Values

The static Keyword

Mutation vs Reassignment, and final

Java Collections: List, Set, Map

Key Takeaways

Open this lesson

The `static` Keyword

Mutation vs Reassignment, and `final`

The `static` Keyword

Mutation vs Reassignment, and `final`