下面这个Java程序为什么用到了构造器和get set方法，只用一个不可以吗

class Apple
{
private String color;
private double weight;
public Apple(){}
public Apple(String color, double weight)
{
this.color=color;
this.weight=weight;

}
public String getColor() {
return color;
}
public void setColor(String color) {
this.color = color;
}
public double getWeight() {
return weight;
}
public void setWeight(double weight) {
this.weight = weight;
}

public String toString()
{
return "一个苹果的颜色是；"+color+"，重量是"+weight;

}
}
public class ToStringTest {
public static void main(String[] args)
{
Apple a=new Apple("红色",58);
System.out.println(a);
}
}

什么时候用构造器 什么时候用get set方法 什么时候全部都用

相关了解……