[HD Pg 0, Sec. 12.5.4 - using-predicate-with-arraylist]

[flex567]

What imports are used for the following code ?

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ArrayList < Integer > iList = new ArrayList < >(); 
iList.addAll( Arrays.asList( 1, 2, 3, 4, 5, 6)); 
Predicate < Integer > p = x-> x% 2 == 0; 
iList.removeIf( p); 
System.out.println( iList);

How would this code look like if the Lambda expression woudn't be used?

[admin]

Replace this line: Predicate < Integer > p = x-> x% 2 == 0;
With:

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Predicate < Integer > p = new Predicate<Integer>(){
          public boolean test(Integer x){
            return x% 2 == 0; 
          }
      };

Equivalency of lambda and anonymous inner class is explained in 12.5.1 and the usage of Predicate without lambda is shown in 12.5.3.

[flex567]

In the Car Mall example we are not instantiating a Predicate object.
Could we use the Predicate object in the Car Mall example like we are using it here?

There wasn't much in the book about Predicate class, only about the Predicate interface and its methods. Is that becuase it belongs more to the next level exam?

[admin]

1. There is no class named Predicate. There is just an interface named Predicate. Just near the begining of section 12.5.3, the book says, "Filtering through a list of objects is such a common requirement in Java application that the Java standard library includes a generic interface for this purpose - java.util.function.Predicate.".

2. Yes, you can use a Predicate object in the CarMall example. Not sure if you have gone through Section 12.5.3 carefully but it explains exactly the question that you have asked here. It shows exactly how you can replace CarFilter of CarMall example with a Predicate.

3. OCAJP exam requires you to know only the Predicate interface and its usage with ArrayList. The book discusses all that you need to know about this for the exam in Section 12.5.3 and 12.5.4. So, go through these two sections very carefully.

[flex567]

I thought there is a class named Preciate because of: Predicate <Integer> p = new Predicate<Integer>(){...

[admin]

It is instantiating an anonymous class that implements Predicate interface. The concept of anonymous class is explained in section 9.2.7.

[flex567]

This is how I implemented it:

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import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;

public class Lambda1 {

    public static void main( String[] args) {


		Predicate <Car> p = new Predicate<Car>(){
          public boolean test(Car c){
            return c.company.equals(" Honda");
          }
		};
		
		// Lambda Expression
       CarMall cm = new CarMall();
       List < Car > carsByCompany = cm.showCars(p);
	   
	   System.out.println( carsByCompany);
		
    }
}

class Car {

    String company;
    int year;
    double price;
    String type;

    Car( String c, int y, double p, String t){
        this.company = c;
        this.year = y;
        this.price = p;
        this.type = t;
    }

    public String toString(){ return "(" + company +" "+ year +")"; }
}

class CarMall {
    List<Car> cars = new ArrayList< >();

    CarMall() {
        cars.add(new Car(" Honda", 2012, 9000.0, "HATCH"));
        cars.add(new Car(" Honda", 2018, 17000.0, "SEDAN"));
        cars.add(new Car(" Toyota", 2014, 19000.0, "SUV"));
        cars.add(new Car(" Ford", 2014, 13000.0, "SPORTS"));
        cars.add(new Car(" Nissan", 2017, 8000.0, "SUV"));
    }

    List <Car> showCars(Predicate<Car> cf){
        ArrayList <Car> carsToShow = new ArrayList < >();

        for( Car c : cars){
            if( cf.test(c)) 
                carsToShow.add(c);
        }
        return carsToShow;
    }	
}

Can lambda be used without predicate interface?

[admin]

I don't see any lambda expression being used in your code.
Yes, of course, lambda expression can be used without Predicate interface. Code on Page 366 shows how -

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public class TestClass{
  public static void main(String[] args) {
     CarMall cm = new CarMall();
     List<Car> carsByCompany = cm.showCars(c -> c.company.equals("Honda"));
     System.out.println(carsByCompany);
  }
}

[flex567]

I don't see any lambda expression being used in your code.

My code was created because of this post:

Code: Select all

2. Yes, you can use a Predicate object in the CarMall example. Not sure if you have gone through Section 12.5.3 carefully but it explains exactly the question that you have asked here. It shows exactly how you can replace CarFilter of CarMall example with a Predicate. 

[OCAJO1]

As you have probably guessed, the lambda expression returns true if an element is even or not.

I think the point will be a lot more clear if 'or not' is removed.

[admin]

Right.