Java Unit Test Code

LoanTest.java
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@ExtendWith(MockitoExtension.class)
class LoanTest {

    /* commit
     * CheckOverdueDate*
     */

    ILoan loan;
    int loanId;
    Date dueDate;

    @Test
    void CommitANonPendingLoan() {
        //Arrange

        //Act
        RuntimeException exception = loan.commit(loanId, dueDate);
        //Assert
        assertThrows(RuntimeException.class, exception);
    }
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Loan.java
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public class Loan implements Serializable, ILoan {

    private int loanId;
    private IBook book;
    private IPatron patron;
    private Date dueDate;
    private LoanState state;


    public Loan(IBook book, IPatron patron) {
        this.book = book;
        this.patron = patron;

        this.state = LoanState.PENDING;
    }

    @Override
    public void commit(int loanId, Date dueDate) {
        if (state != LoanState.PENDING) {
            throw new RuntimeException("Cannot commit a non PENDING loan");
        }
        this.loanId = loanId;
        this.dueDate = dueDate;
        this.state = LoanState.CURRENT;

        patron.takeOutLoan(this);
        book.borrowFromLibrary();
    }

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ILoan.java
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package library.entities;

import java.util.Date;

public interface ILoan {

    enum LoanState {
        PENDING, CURRENT, OVER_DUE, DISCHARGED
    };

    int getId();

    boolean checkOverDue(Date currentDate);

    boolean isOverDue();

    Date getDueDate();

    IPatron getPatron();

    IBook getBook();

    void commit(int loanId, Date dueDate);

    void discharge(boolean isDamaged);

}