Java无法推断“类”的类型参数

Question

Hello so I have been trying to figure this problem out for a bit now and I googled. I have found posts on here with the same problem, but they didn't really help me.

In my JUnit test the call for PaymentBatchProcessor checkProcessor = new PaymentBatchProcessor<>();

causes "Cannot infer type arguments for PaymentBatchProcessor<>"

I just learned about Generics and that's why I'm having such a hard time implementig it. I understand the general concept of Generics.

    public class PaymentBatchProcessor <T extends Payment> {

        // Variables
        private T payment;

        List <T> listOfPayments;

        // Constructor
        public PaymentBatchProcessor(T payment) {
            this.payment = payment;
            this.listOfPayments = new ArrayList<>();
        }

        // add method
        public void add(T payment) {

            this.listOfPayments.add(payment);
        }

        // getMax method
        public double getMax () {
            // Placeholder
            double maxAmount = 0.0;

            for( T payment : listOfPayments) {
                // displaying each payment to console
                System.out.println(payment);

                // If current payment is more then current maxAmount
                //      assign new highest amount
                if(payment.getAmount() > maxAmount) {
                    maxAmount = payment.getAmount();
                }
            }
            // Return highest amount
            return maxAmount;   
        }// END OF getMax()


        // getTotal method
        public double getTotal() {

            // Accumulator
            double total = 0.0;

            // Add each payment amount to total
            for( T payment : listOfPayments) {
                total +=  payment.getAmount();
            }

            return total;
        }// END OF getTotal()

        // getSize method
        public int getSize() {
            // Return size of list
            return listOfPayments.size();
        }

}// END OF PAYMENTBATCHPROCESSOR



//Interface
public interface Payment {

        public  double getAmount();
        public void setAmount(double amount);


//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


    // Check class
    public class Check implements Payment{
        // Variable
        private double amount;

        // Constructor
        public Check (double amount) {
            this.amount = amount;
        }

        // Getter and Setter
        public double getAmount() {
            return this.amount;
        }

        public void setAmount(double amount) {
            this.amount = amount;
        }
    }// END OF CHECK

//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

    public class IOU implements Payment {
        // Variable
        private double amount;

        // Constructor
        public IOU (double amount) {
            this.amount = amount;
        }

        // Getter and Setter
        public double getAmount() {
            return this.amount;
        }

        public void setAmount(double amount) {
            this.amount = amount;
        }
    }// END OF IOU
}
// +++++++++++++ PROBLEM AREA +++++++++++++++++++++++++++++++++++++++++++++++++

import static org.junit.Assert.*;
import java.util.ArrayList;
import org.junit.Test;



public class TestGenerics
{

    @Test
    public void testProcessorAsCheck()
    {
        PaymentBatchProcessor<Check> checkProcessor = new PaymentBatchProcessor<>();

        checkProcessor.add( new Check(5.00) );
        checkProcessor.add (new Check(10.00) );

        assertEquals(15, checkProcessor.getTotal(), 2);
        assertEquals(10, checkProcessor.getMax(), 2);
    }

    @Test
    public void testProcessorAsIOU()
    {
        PaymentBatchProcessor<IOU> processor = new PaymentBatchProcessor<>();

        processor.add( new IOU(22.54) );
        processor.add( new IOU(22.55) );

        assertEquals(45.09, processor.getTotal(), 2);
        assertEquals(22.55, processor.getMax(), 2);
    }

    @Test
    public void testProcessorAsPayment()
    {
        Payment iou = new IOU(11.22);
        Payment iou2 = new Check(22.11);

        PaymentBatchProcessor<Payment> processor = new PaymentBatchProcessor<>();

        processor.add(iou);
        processor.add(iou2);

        assertEquals(33.33, processor.getTotal(), 2);
        assertEquals(22.11, processor.getMax(), 2);
    }

    @Test
    public void testProcessorAsPaymentWithEmptyList()
    {
        PaymentBatchProcessor<Payment> processor = new PaymentBatchProcessor<>();

        assertEquals(0, processor.getTotal(), 2);
        assertNull(null, processor.getMax());
    }

    @Test
    public void testProcessorHelperAsPayment()
    {
        ArrayList<Payment> list = new ArrayList<Payment>();

        list.add( new Check(10.00) );
        list.add( new Check(5.00) );
        list.add( new IOU(1.00) );

        assertEquals(10, PaymentProcessorHelper.<Payment> getMax(list).getAmount(), 2);
        assertEquals(16, PaymentProcessorHelper.<Payment> getSum(list), 2);
    }

    @Test
    public void testProcessorHelperAsPaymentEmptyList()
    {
        ArrayList<Payment> list = new ArrayList<Payment>();

        assertNull(PaymentProcessorHelper.<Payment> getMax(list));
        assertEquals(0, PaymentProcessorHelper.<Payment> getSum(list), 2);
    }

    @Test
    public void testProcessorHelperAsCheck()
    {
        ArrayList<Check> list = new ArrayList<Check>();

        list.add( new Check(10.00) );
        list.add( new Check(5.00) );

        assertEquals(10, PaymentProcessorHelper.<Check> getMax(list).getAmount(), 2);
        assertEquals(15, PaymentProcessorHelper.<Check> getSum(list), 2);
    }

    @Test
    public void testProcessorHelperAsIOU()
    {
        ArrayList<IOU> list = new ArrayList<IOU>();

        list.add( new IOU(11.22) );
        list.add( new IOU(22.11) );

        assertEquals(22.11, PaymentProcessorHelper.<IOU> getMax(list).getAmount(), 2);
        assertEquals(33.11, PaymentProcessorHelper.<IOU> getSum(list), 2);
    }

}

Answer 1

For PaymentBatchProcessor you have defined a constructor that takes a Payment as an argument, but in the tests you try to use a no-arguments constructor new PaymentBatchProcessor<>() , which doesn't exist.

You either need to define a no-arguments constructor or provide an argument to the constructors in your tests.

And I'm also trying to make getMax and getTotal a generic method

Based on the code in your question I don't really understand why you would want to do that.

I tried making them public static < T >

I think you have misunderstood something about generics (and also the static modifier) here.

It doesn't look like getMax and getTotal should ever return anything other than double and they don't take any arguments so there is no issue of handling different types of inputs.
And you can't make those methods static because they operate on instance variables (not class variables).