Spring Dependency Injection with Example

package com.geeksforgeeks.org;

import com.geeksforgeeks.org.IGeek;
import org.springframework.beans.factory.annotation.Autowired;

public class GFG {

    // The object of the interface IGeek
    private IGeek geek;

    // Setter method for property geek with @Autowired annotation
    @Autowired
    public void setGeek(IGeek geek) {
        this.geek = geek;
    }
}

package com.geeksforgeeks.org;

import com.geeksforgeeks.org.IGeek;

public class GFG {

    // The object of the interface IGeek
    private IGeek geek;

    // Constructor to set the CDI
    public GFG(IGeek geek) {
        this.geek = geek;
    }
}

interface IEngine {
    String EMISSION_NORMS = "BSIV";
    String importOrigin();
    double cost();
}

public class ToyotaEngine implements IEngine {
    String company;
    double cost;
    public double getCost() { return cost; }

    public void setCost(double cost) { cost = this.cost; }

    public String getCompany() { return company; }

    public void setCompany(String company)
    {
        this.company = company;
    }

    @Override public String importOrigin()
    {
        return "Japan";
    }

    @Override public double cost() { return cost; }
    @Override public String toString()
    {
        return "This is Engine object from: " + company;
    }
}

public class Tyres {

    String name;
    String place;
    String message;

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public String getPlace() { return place; }
    public void setPlace(String place)
    {
        this.place = place;
    }
    public String getMessage() { return message; }
    public void setMessage(String message)
    {
        this.message = message;
    }

    @Override public String toString()
    {
        return "This is Tyre object: " + name + " " + place
            + " " + message;
    }
}

public class Vehicle {

    IEngine engine;
    Tyres tyre;

    public Tyres getTyre() { return tyre; }

    public void setTyre(Tyres tyre)
    {
        System.out.println("tyre instantiated via setter");
        this.tyre = tyre;
    }

    public Vehicle(IEngine engine, Tyres tyre)
    {
        System.out.println("instantiated via constructor");
        this.engine = engine;
        this.tyre = tyre;
    }

    public Vehicle() {}
    public IEngine getEngine() { return engine; }
    public void setEngine(IEngine engine)
    {
        System.out.println("instantiated via setter");
        this.engine = engine;
    }

    @Override public String toString()
    {
        return engine + " " + tyre;
    }

    public static void main(String a[])
    {
        ApplicationContext rootctx
            = new ClassPathXmlApplicationContext(
                "springContext.xml");

        // Instantiating the obj1 via Constructor DI
        Vehicle obj1 = (Vehicle)rootctx.getBean(
            "InjectwithConstructor");

        // Instantiating the obj1 via Setter DI
        Vehicle obj2
            = (Vehicle)rootctx.getBean("InjectwithSetter");

        System.out.println(obj1);
        System.out.println(obj2);
        System.out.println(obj1 == obj2);
    }
}