Object, Class, Math and System in Java

1. Object Class

• Package: java.lang
• Role: The root class of all classes in Java. Every class inherits from Object.
• Key Methods:
• toString() Returns a string representation of the object.
• equals(Object obj) Checks if two objects are equal.
• hashCode() Returns a hash code for the object.
• getClass() Returns the runtime Class object of the object.
• clone() Creates and returns a copy of the object.
• notify(), notifyAll(), wait() Used for thread communication.

Example:

class Person {
    String name;
    Person(String name) { this.name = name; }
    @Override
    public String toString() { return "Person[name=" + name + "]"; }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Alice");
        System.out.println(p); // Person[name=Alice]
    }
}

class Person {
    String name;
    Person(String name) { this.name = name; }
    @Override
    public String toString() { return "Person[name=" + name + "]"; }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Alice");
        System.out.println(p); // Person[name=Alice]
    }
}

2. Class Class

• Package: java.lang
• Role: Represents the metadata (runtime type information) of a class or interface.
• How to Obtain a Class Object:
• String s = "Hello"; Class<?> c1 = s.getClass(); // via object
• Class<?> c2 = String.class; // via class literal
• Class<?> c3 = Class.forName("java.lang.String"); // via name
• Key Methods:
• getName() Returns class name.
• getSuperclass() Returns superclass.
• getInterfaces() Returns implemented interfaces.
• getMethods() Returns public methods.
• isInterface() Checks if it’s an interface.
• isArray() Checks if it’s an array type.

Example:

public class Main {
    public static void main(String[] args) throws Exception {
        Class<?> cls = Class.forName("java.lang.String");
        System.out.println("Class Name: " + cls.getName());
        System.out.println("Superclass: " + cls.getSuperclass().getName());
    }
}

public class Main {
    public static void main(String[] args) throws Exception {
        Class<?> cls = Class.forName("java.lang.String");
        System.out.println("Class Name: " + cls.getName());
        System.out.println("Superclass: " + cls.getSuperclass().getName());
    }
}

3. Math Class

• Package: java.lang
• Role: Provides static methods for mathematical operations.
• Key Methods:
• Math.abs(x) Absolute value.
• Math.sqrt(x) Square root.
• Math.pow(a,b) Power calculation.
• Math.max(a,b) / Math.min(a,b) Max/min values.
• Math.random() Random double (0.0–1.0).
• Math.round(x) Rounds value to nearest integer.

Example:

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.sqrt(16));   // 4.0
        System.out.println(Math.pow(2, 3)); // 8.0
        System.out.println(Math.random());  // e.g., 0.734
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.sqrt(16));   // 4.0
        System.out.println(Math.pow(2, 3)); // 8.0
        System.out.println(Math.random());  // e.g., 0.734
    }
}

4. System Class

• Package: java.lang
• Role: Provides system-related utility methods and fields.
• Important Fields:
  • System.in → Standard input stream.
  • System.out → Standard output stream.
  • System.err → Standard error stream.
• Key Methods: Method Purpose
• System.currentTimeMillis() Current time in milliseconds.
• System.arraycopy() Copies array elements.
• System.exit(status) Terminates the JVM.
• System.gc() Suggests garbage collection.
• System.getProperty(key) Gets system property.

Example:

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello World!");
        long time = System.currentTimeMillis();
        System.out.println("Time in ms: " + time);
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello World!");
        long time = System.currentTimeMillis();
        System.out.println("Time in ms: " + time);
    }
}

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