10 Java Code Snippets to Test Your Output Prediction Skills

 

🧠 10 Java Output Questions That Will Sharpen Your Mind

One of the best ways to strengthen your Java skills is by predicting what a program prints without running it. These snippets test your understanding of concepts like operators, memory, method overloading, strings, and more. Ready for a brain workout?

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✅ 1. String Pool Puzzle

public class Test1 {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = new String("hello");
        System.out.println(s1 == s2);
        System.out.println(s1.equals(s2));
    }
}

🟡 Output:

false
true

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✅ 2. Post-Increment Confusion

public class Test2 {
    public static void main(String[] args) {
        int x = 5;
        int y = x++;
        System.out.println(x);
        System.out.println(y);
    }
}

🟡 Output:

6
5

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✅ 3. Static vs Instance Method

class Demo {
    static void print() {
        System.out.println("Static");
    }
    void show() {
        System.out.println("Instance");
    }
    public static void main(String[] args) {
        Demo d = null;
        d.print();
        // d.show(); // Uncomment this line and see what happens
    }
}

🟡 Output:

Static

(Calling static method via null object is allowed, but not instance methods.)

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✅ 4. String Immutability Trap

public class Test4 {
    public static void main(String[] args) {
        String s = "abc";
        s.concat("def");
        System.out.println(s);
    }
}

🟡 Output:

abc

(Strings are immutable. concat doesn’t change the original unless assigned.)

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✅ 5. Switch Without Break

public class Test5 {
    public static void main(String[] args) {
        int day = 2;
        switch(day) {
            case 1: System.out.println("Mon");
            case 2: System.out.println("Tue");
            case 3: System.out.println("Wed");
        }
    }
}

🟡 Output:

Tue
Wed

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✅ 6. Overloading Mystery

public class Test6 {
    void test(int a) {
        System.out.println("int");
    }
    void test(Integer a) {
        System.out.println("Integer");
    }
    public static void main(String[] args) {
        new Test6().test(10);
    }
}

🟡 Output:

int

(Primitive match is preferred over wrapper type.)

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✅ 7. Array Printing

public class Test7 {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        System.out.println(arr);
    }
}

🟡 Output:

[I@15db9742

(Default toString() on array shows object reference. Use Arrays.toString() for readable output.)

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✅ 8. Final Parameters

public class Test8 {
    public static void main(final String[] args) {
        args[0] = "Changed";
        System.out.println(args[0]);
    }
}

🟡 Output:

Changed

(You can change contents of final reference, not the reference itself.)

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✅ 9. Null Argument Overloading

public class Test9 {
    void print(Object o) {
        System.out.println("Object");
    }
    void print(String s) {
        System.out.println("String");
    }
    public static void main(String[] args) {
        new Test9().print(null);
    }
}

🟡 Output:

String

(Most specific method is chosen when calling with null.)

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✅ 10. Wrapper Caching

public class Test10 {
    public static void main(String[] args) {
        Integer a = 100;
        Integer b = 100;
        Integer x = 200;
        Integer y = 200;
        System.out.println(a == b);
        System.out.println(x == y);
    }
}

🟡 Output:

true
false

(Integer caching works for values between -128 and 127.)

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💡 Final Thoughts

These snippets aren't just trivia — they reveal how Java handles memory, types, and execution flow. Try modifying each example slightly to see how behavior changes!

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