Java自学笔记03—对象与类（下） - 黑马程序员技术交流社区

public class HelloJava
{
public static void main(String[] args)
{
System.out.println("Testing tripleValue:");
double percent = 10;
System.out.println("Before: percent=" + percent);
tripleValue(percent);
System.out.println("After: percent=" + percent);
System.out.println("\nTesting tripleSalary:");
Employee harry = new Employee("Harry", 50000);
System.out.println("Before: salary=" + harry.getSalary());
tripleSalary(harry);
System.out.println("After: salary=" + harry.getSalary());
System.out.println("\nTesting swap:");
Employee a = new Employee("Alice", 70000);
Employee b = new Employee("Bob", 60000);
System.out.println("Before: a=" + a.getName());
System.out.println("Before: b=" + b.getName());
swap(a, b);
System.out.println("After: a=" + a.getName());
System.out.println("After: b=" + b.getName());
}
public static void tripleValue(double x)
{
x = 3 * x;
System.out.println("End of method: x=" + x);
}
public static void tripleSalary(Employee x)
{
x.raiseSalary(200);
System.out.println("End of method: salary=" + x.getSalary());
}
public static void swap(Employee x, Employee y)
{
Employee temp = x;
x = y;
y = temp;
System.out.println("End of method: x=" + x.getName());
System.out.println("End of method: y=" + y.getName());
}
}
class Employee
{
private String name;
private double salary;
public Employee(String n, double s)
{
name = n;
salary = s;
}
public String getName()
{
return name;
}
public double getSalary()
{
return salary;
}
public void raiseSalary(double byPercent)
{
double raise = salary * byPercent / 100;
salary += raise;
}
}

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对象构造
如果多个方法有相同的名字，不同的参数，便产生了重载。
如果在构造器中没有显示地给域赋予初值，那么就会被自动地赋为默认值：数值为0，布尔值为false，对象引用为null。
如果类中没有构造器，系统会提供一个无参数的构造器，这个构造器将所有实力域设置为默认值。如果类中有构造器，系统不会提供构造器。
如果构造器的参数名与实例域名相同，可以采用this指针完成初始化，例如：
public Employee(String name, double salary)
{
this.name = name;
this.salary = salary;
}
如果构造器的第一个语句形如this(…)，这个构造器将调用同一个类的另一个构造器。这种处理方式使得对公共构造器代码部分只编写一次即可。
还可以使用初始化块初始化域，初始化块先于构造器执行。

import java.util.*;
public class HelloJava
{
public static void main(String[] args)
{
Employee[] staff = new Employee[3];
staff[0] = new Employee("Harry", 40000);
staff[1] = new Employee(60000);
staff[2] = new Employee();
for (Employee e : staff)
System.out.println("name=" + e.getName() + ",id=" + e.getId() + ",salary=" + e.getSalary());
}
}
class Employee
{
private static int nextId;
private int id;
private String name = "";
private double salary;
static
{
Random generator = new Random();
nextId = generator.nextInt(10000);
}
{
id = nextId;
nextId++;
}
public Employee(String n, double s)
{
name = n;
salary = s;
}
public Employee(double s)
{
this("Employee #" + nextId, s);
}
public Employee()
{
}
public String getName()
{
return name;
}
public double getSalary()
{
return salary;
}
public int getId()
{
return id;
}
}

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包
标准Java包处于java和javax包层次中。使用包的主要原因是确保类名的唯一性。从编译器的角度看，嵌套的包之间没有任何关系。
使用import语句导入特定类或整个包，使用import static …导入静态方法和静态域。
使用package语句将类放入包中。

import com.horstmann.corejava.*;
import static java.lang.System.*;
public class HelloJava
{
public static void main(String[] args)
{
Employee harry = new Employee("Harry Hacker", 50000, 1989, 10, 1);
harry.raiseSalary(5);
out.println("name=" + harry.getName() + ",salary=" + harry.getSalary());
}
}

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package com.horstmann.corejava;
import java.util.*;
public class Employee
{
private String name;
private double salary;
private Date hireDay;
public Employee(String n, double s,int year, int month, int day)
{
name = n;
salary = s;
GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
hireDay = calendar.getTime();
}
public String getName()
{
return name;
}
public double getSalary()
{
return salary;
}
public Date getHireDay()
{
return hireDay;
}
public void raiseSalary(double byPercent)
{
double raise = salary * byPercent / 100;
salary += raise;
}
}

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没有指定public或private的类，方法，域可以被同一个包中的所有方法访问。