In Java, declaring a class abstract is useful code example

Example 1: abstract class in java

Sometimes we may come across a situation where we cannot provide 
implementation to all the methods in a class. We want to leave the 
implementation to a class that extends it. In such case we declare a class
as abstract.To make a class abstract we use key word abstract. 
Any class that contains one or more abstract methods is declared as abstract. 
If we don’t declare class as abstract which contains abstract methods we get 
compile time error.
  
  1)Abstract classes cannot be instantiated
  2)An abstarct classes contains abstract method, concrete methods or both.
  3)Any class which extends abstarct class must override all methods of abstract
    class
  4)An abstarct class can contain either 0 or more abstract method.

Example 2: What are abstract methods in java

An abstract method is the method which does’nt have any body. 
Abstract method is declared with
keyword abstract and semicolon in place of method body.

  public abstract void ();
Ex : public abstract void getDetails();
It is the responsibility of subclass to provide implementation to 
abstract method defined in abstract class

Example 3: how to make abstract method in java

public abstract class Account {		//abstract class //perent class
    protected int accountNumber;
    protected Customer customerObj;
    protected double balance;
  	//constructor
  	public Account(int saccountNumber, Customer scustomerObj,double sbalance){
        accountNumber = saccountNumber;
        customerObj = scustomerObj;
        balance = sbalance;
    }
  	// abstract Function
    public abstract boolean withdraw(double amount); 
}   

public class SavingsAccount extends Account { // child class
    private double minimumBalance;
  	// constructor
    public SavingsAccount(int saccountNumber, Customer scustomerObj, double sbalance, double sminimumBalance) {
        super(saccountNumber, scustomerObj, sbalance);
        minimumBalance = sminimumBalance;
    }
	// Implementation of abstract function in child class
    public boolean withdraw(double amount) {
        if (balance() > minimumBalance && balance() - amount > minimumBalance) {
            super.setBalance(balance() - amount);
            return true;
        } else {
            return false;
        }
    }
}