Java Software Engineer

☛ Method Overloading:

In Method Overloading, Methods of the same class shares the same name but each method must have different number of parameters or parameters having different types and order.

Method Overloading is to “add” or “extend” more to method’s behavior.

It is a compile time polymorphism.

The methods must have different signature.

It may or may not need inheritance in Method Overloading.

Let’s take a look at the example below to understand it better.

class Adder {

Static int add(int a, int b)

{

return a+b;

}

Static double add( double a, double b)

{

return a+b;

}

public static void main(String args[])

{

System.out.println(Adder.add(11,11));

System.out.println(Adder.add(12.3,12.6));

}}

☛ Method Overriding:

In Method Overriding, sub class have the same method with same name and exactly the same number and type of parameters and same return type as a super class.

Method Overriding is to “Change” existing behavior of method.

It is a run time polymorphism.

The methods must have same signature.

It always requires inheritance in Method Overriding.

Let’s take a look at the example below to understand it better.

class Car {

void run(){

System.out.println(“car is running”);

}

Class Audi extends Car{

void run()

{

System.out.prinltn(“Audi is running safely with 100km”);

}

public static void main( String args[])

{

Car b=new Audi();

b.run();

}

}

In Java, each thread has its own stack, including its own copy of variables it can access. When the thread is created, it copies the value of all accessible variables into its own stack. The volatile keyword basically says to the JVM “Warning, this variable may be modified in another Thread”.

In all versions of Java, the volatile keyword guarantees global ordering on reads and writes to a variable. This implies that every thread accessing a volatile field will read the variable’s current value instead of (potentially) using a cached value.

In Java 5 or later, volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex.

Using volatile may be faster than a lock, but it will not work in some situations. The range of situations in which volatile is effective was expanded in Java 5; in particular, double-checked locking now works correctly.

The volatile keyword is also useful for 64-bit types like long and double since they are written in two operations. Without the volatile keyword you risk stale or invalid values.

One common example for using volatile is for a flag to terminate a thread. If you’ve started a thread, and you want to be able to safely interrupt it from a different thread, you can have the thread periodically check a flag (i.e., to stop it, set the flag to true). By making the flag volatile, you can ensure that the thread that is checking its value will see that it has been set to true without even having to use a synchronized block. For example:

public class Foo extends Thread {

private volatile boolean close = false;

public void run() {

while(!close) {

// do work

}

}

public void close() {

close = true;

// interrupt here if needed

}

}

In Java, Strings are immutable and are stored in the String pool. What this means is that, once a String is created, it stays in the pool in memory until being garbage collected. Therefore, even after you’re done processing the string value (e.g., the password), it remains available in memory for an indeterminate period of time thereafter (again, until being garbage collected) which you have no real control over. Therefore, anyone having access to a memory dump can potentially extract the sensitive data and exploit it.

In contrast, if you use a mutable object like a character array, for example, to store the value, you can set it to blank once you are done with it with confidence that it will no longer be retained in memory.

In general, String s = "Test" is more efficient to use than String s = new String("Test").

In the case of String s = "Test", a String with the value “Test” will be created in the String pool. If another String with the same value is then created (e.g., String s2 = "Test"), it will reference this same object in the String pool.

However, if you use String s = new String("Test"), in addition to creating a String with the value “Test” in the String pool, that String object will then be passed to the constructor of the String Object (i.e., new String("Test")) and will create another String object (not in the String pool) with that value. Each such call will therefore create an additional String object (e.g., String s2 = new String("Test") would create an addition String object, rather than just reusing the same String object from the String pool).

If a collection backs another, it means that changes in one are reflected in the other and vice-versa.

For example, suppose we wanted to create a whitelist function that removes invalid keys from a Map. This is made far easier with Map.keySet, which returns a set of keys that is backed by the original map. When we remove keys from the key set, they are also removed from the backing map:

public static <K, V> Map<K, V> whitelist(Map<K, V> map, K... allowedKeys) {

Map<K, V> copy = new HashMap<>(map);

copy.keySet().retainAll(asList(allowedKeys));

return copy;

}

retainAll writes through to the backing map, and allows us to easily implement something that would otherwise require iterating over the entries in the input map, comparing them against allowedKey, etcetera.

Note, it is important to consult the documentation of the backing collection to see which modifications will successfully write through. In the example above, map.keySet().add(value) would fail, because we cannot add a key to the backing map without a value.

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. Let’s take a look at the example below to understand it better.

class Car {

void run()

{

System.out.println(“car is running”);

}

}

class Audi extends Car {

void run()

{

System.out.prinltn(“Audi is running safely with 100km”);

}

public static void main(String args[])

{

Car b= new Audi(); //upcasting

b.run();

}

}