题目是关于linkedlist， 每次测试运行显示 Payment是null
public class Payment {
public String fromPerson; // name of Person paying
public String toPerson; // name of Person receiving payment
public double amount; // amount payed
public Timestamp timestamp; // unique time stamp (provided at the time of creation)
public Payment next; // reference to next payment in appropriate Ledger Linked List
/**
* Basic empty constructor for a payment
*/
public Payment() {
// Provided - not to be changed
this.fromPerson = null;
this.toPerson = null;
this.amount = 0;
this.timestamp = null;
this.next = null;
}
/**
* Payment constructor with all necessary fields to be inserted in Ledger
*
* @param fromPerson is the person paying
* @param toPerson is the person receiving the payment
* @param amount is the amount payed
* @param timestamp is time at creation of payment
* @param next is the next payment
*/
public Payment(String fromPerson, String toPerson, double amount, Payment next) {
// Provided - not to be changed
this.fromPerson = fromPerson;
this.toPerson = toPerson;
this.amount = amount;
this.timestamp = new Timestamp(); // time stamp is automatically set at creation
this.next = next;
}
/**
* A Payment is valid if it is done between registered persons provided in the
* array persons
*
* You may assume that the input array (String[] persons) will
* not be null or empty.
*
* @param persons - array of persons who can make and receive payment
*
* @return true if the Payment is valid based on the criteria listed above,
* false otherwise
*/
public boolean isValid(String[] persons) {
boolean flag1=false;
boolean flag2=false;
for(int i=0;iif(this.fromPerson.toLowerCase().equals(persons[i].toLowerCase()))
flag1=true;
if(this.toPerson.toLowerCase().equals(persons[i].toLowerCase()))
flag2=true;
}
return flag1&&flag2;
}
}
public class Ledger {
public Payment head;
public int id;
public Ledger next;
// add attributes as needed
// needed to implement efficient code
// tested for `advanced' parts
public Payment tail;
/**
* Default constructor
*
* You may modify this constructor if you need to (e.g.
* if you want to initialise extra attributes in the class)
*/
public Ledger() {
head = null;
id = 0;
next = null;
//
// add initialisation of added attributes, if needed
}
/**
* Assign an id to the current Ledger
*
* @param id
*/
public void setId(int id) {
//
// Provided - not to be changed
this.id = id;
}
/**
*
* @return the number of payments in the Ledger
*/
public int size() {
Payment curr=head;
int count=0;
while(curr!=null) {
count++;
curr=curr.next;
}
return count;
}
/**
* Add a payment at the end of the Ledger list
*
* @param payment
*/
public void addPayment(Payment payment) {
// if Payment is null, do nothing
Payment head=payment ;
Payment temp=head;
if(head!=null) {
while(temp.next!=null) {
temp=temp.next;
}
temp.next= new Payment();
}
}
**TEST CASE :**
```java
public void testLedgerSize() {
assertTimeoutPreemptively(Duration.ofMillis(1000), () -> {
String[] fromPerson = { "Eren", "Mikasa", "a", "c", "Armin", "Jean", "Levi", "Sakura", "Naruto", "Erwin",
"Thomas",
"Sasuke", "Ichigo", "Luffy", "Scarlet"
};
String[] toPerson = { "Levi", "a", "Mikasa", "c", "Jean", "Armin", "Sakura", "Levi", "Sasuke", "Erwin", "Scarlet",
"Naruto", "Rukia", "Ichigo", "Thomas"
};
double[] payment = { 10.0, 100.1, 21.0, 55.5, 29.9, 99.9, 100.0, 29.99, 30.0, 24.95, 35.0, 34.49, 99.95, 50,
400 };
// String[] registeredPeople = {"Eren", "Levi", "Mikasa", "Armin", "Erwin",
// "Jean", "Annie", "Reiner", "Hanji", "Sasha"};
// Case 1: empty Ledger
Ledger empty = new Ledger();
assertEquals(0, empty.size());
// Case 2: Non empty Ledger size computed accurately
Ledger nonEmpty = new Ledger();
for (int i = 0; i < fromPerson.length; i++) {
nonEmpty.addPayment(new Payment(fromPerson[i], toPerson[i], payment[i], null));
}
assertEquals(toPerson.length, nonEmpty.size());
// Case 3: Ledger with one Payment only
Ledger singlePayment = new Ledger();
singlePayment.addPayment(new Payment("Eren", "Levi", 100, null));
assertEquals(1, singlePayment.size());
});
currentMethodName = new Throwable().getStackTrace()[0].getMethodName();
}
我有尝试加 constructor， initialize Payment 和 Ledger 让它不为null
Ledger.size() and Ledger.addPayment()工作
怎么说呢，逻辑太混乱了，简单说几个错误的地方。
public Ledger() {
head = null; // head永远是null，下面也没有任何修改head的地方，当然你的size永远是0了。
id = 0;
next = null;
}
public void addPayment(Payment payment) {
// 从你这个add方法实现来看，是把入参的payment当作了head，然后在其最后添加了一个new Payment();
// 这样你必须要payment.size()，才会有结果啊，singlePayment.size()肯定还是0
// 但是我想，或者说一个正常的add方法，目的肯定不是这样的，应该是直接把入参的payment添加到队尾。
// 因此直接把下一行删掉即可。
// Payment head=payment ;
Payment temp=head;
if(head!=null) {
while(temp.next!=null) {
temp=temp.next;
}
temp.next= new Payment();
}
}