package pkgPeople;
import java.io.Serializable;
import java.text.DecimalFormat;
public class Person implements Serializable{
private String name;
private int height;
private int weight;
private BankAccount bankAccount;
private static int personCT=0;
private String noAccount;
//The Default constructor, a method
public Person()
{
personCT = personCT +1;
}
// We have another constructor; this is also a method
public Person(int height,int weight, String name, BankAccount bankAccount)
{
personCT++;
this.name = name;
this.height = height;
this.weight = weight;
}
//Constructor for a person with a bank account
public Person(int height,int weight, String name, int acctID, double balance)
{
personCT++;
this.bankAccount.setAcctID(acctID);
this.bankAccount.setBalance(balance);
this.name = name;
this.height = height;
this.weight = weight;
}
//Mutator
public void setNoAccount() {
this.bankAccount = null;
}
//Accessor
public String getNoAccount() {
return noAccount;
}
//Accessor
public BankAccount getBankAccount() {
return bankAccount;
}
//Mutator
public void setBankAccount(BankAccount bankAccount) {
this.bankAccount = bankAccount;
}
//Accessor method, get...
public String getName() {
return name;
}
public void setNoAccount(String noAccount) {
this.noAccount = noAccount;
}
public static int getPersonCT()
{
return personCT;
}
//Mutator method, set...
public void setName(String name) {
this.name = name;
}
public int getWeight() {
return weight;
}
public void setWeight(int weight) {
this.weight = weight;
}
//Method to set the height
public void setHeight(int ht)
{
height = ht;
}
public int getHeight()
{
return height;
}
//Method to calculate Body Mass Index
// BMI equals (Weight in Kg)/(height in meters squared)
public double calcBMI()
{// all the variables in the next line are local variables
double num, den, heightInCm, heightInMeters,bmi;
//The numerator is calculated
num = weight * (1/2.2); //1 pound = (1/2.2)Kg
heightInCm = height * 2.54; // 1 inch = 2.54 cm
heightInMeters = heightInCm/100; // 100cm = meter
den = Math.pow(heightInMeters, 2);
bmi = num/den;
return bmi;
}
//Method to determine the category for BMI
public String findBMICategory()
{
double bmi;//local variable, it only exists when the method is running
bmi = calcBMI(); // bmi = this.calcBMI(); would have been ok
if (bmi>=30)
{
return "Obese";
}
else if ((bmi>=25) && (bmi<30))
{
return "Overweight";
}
else if ((bmi>=18.5) && (bmi<25))
{
return "Normal Weight";
}
else
{
return "Under Weight";
}
}//end of findBMICategory
public String toString()
{
DecimalFormat fmt = new DecimalFormat("###");
DecimalFormat fmt1 = new DecimalFormat("#####");
DecimalFormat fmt2 = new DecimalFormat("##.00");
DecimalFormat fmt3 = new DecimalFormat("#####.##");
String noAccount = "NO BANK ACCOUNT";
{
return "The name is " + name + ".\n"
+ "The height is " + fmt.format(height) + " inches.\n"
+ "The weight is " + fmt.format(weight) + " pounds.\n"
+ "The BMI is " + fmt2.format(calcBMI()) + ".\n"
+ "The " + fmt1.format(bankAccount.getAcctID()) + "bank account has " + fmt3.format(bankAccount.getBalance()) +"dollars in it.\n"
+ "Classification: " + findBMICategory()+ "\n\n";
}
这是我要跑步的主要内容...
package pkgPeople;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.ArrayList;
public class MakePeople {
//Create people and output them to a file
public static void main(String[] args) throws IOException{
FileOutputStream file = new FileOutputStream("friends.dat");
ObjectOutputStream outStream = new ObjectOutputStream(file);
ArrayList <Person> people = new ArrayList<Person>();
people.add(new Person(69, 165, "David Jones", (new BankAccount(12345, 2000.00))));
people.add(new Person(62, 122, "Mary Decker", (new BankAccount(67890, 3020.50))));
people.add(new Person(72, 250, "Jack Denner", (new BankAccount(33366, 2045.88))));
people.add(new Person(66, 108, "Linda Hall", (new BankAccount(54321, 345.67))));
people.add(new Person(68, 175, "Jim Broke", (new BankAccount(55555, 1.00))));
people.add(new Person(67, 134, "Sally Kroll", (new BankAccount(44444, 129.00))));
people.add(new Person(73, 200, "Fred Baker", (new BankAccount(66666, 666.66))));
people.add(new Person(61, 110, "Jane Harris", (new BankAccount(22222, 211.99))));
int index;
//Print the people
for(index = 0; index < people.size(); index++)
{
System.out.println(people.get(index));
}
outStream.writeObject(people); //can serialize this way
/*
//Could serialize as below
for(index = 0; index < people.size(); index++)
{
outStream.writeObject(people.get(index));
}
*/
outStream.close();
file.close();
}
}
好的，这是我的People课。尝试运行main时，我在第154行收到以下错误。
Exception in thread "main" java.lang.NullPointerException
at pkgPeople.Person.toString(Person.java:154)
at java.lang.String.valueOf(Unknown Source)
at java.io.PrintStream.println(Unknown Source)
at pkgPeople.MakePeople.main(MakePeople.java:32)
现在，这是BankAccount类，我正在尝试从中提取AcctID和余额。
package pkgPeople;
import java.io.Serializable;
public class BankAccount implements Serializable{
long acctID;
double balance;
String dateCreated;
public BankAccount(long acctID, double balance)
{
this.acctID = acctID;
this.balance = balance;
}
public long getAcctID() {
return acctID;
}
public void setAcctID(long acctID) {
this.acctID = acctID;
}
public double getBalance() {
return balance;
}
public void setBalance(double balance) {
this.balance = balance;
}
public String getDateCreated() {
return dateCreated;
}
public void setDateCreated(String dateCreated) {
this.dateCreated = dateCreated;
}
}
有什么想法为什么会这样？这是构造函数错误吗？
谢谢！
最佳答案
我打赌bankAccount为空。
在此构造函数中：
// We have another constructor; this is also a method
public Person(int height,int weight, String name, BankAccount bankAccount)
{
personCT++;
this.name = name;
this.height = height;
this.weight = weight;
}
您不要执行以下操作（我敢打赌您想这样做）：
this.bankAccount = bankAccount;
关于java - java:NullPointerException，需要帮助，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/3852573/