assert method

assertを、入力値チェック、出力値チェック、コンストラクタチェックなどに使う
assertはデフォルト無効なため、java -eaオプションで実行すること

class Item{
    private String name;
    private int price;

    public Item(String name, int price) {
        this.name = name;
        this.price = price;
        assert price > 100 : "constructor price over error";
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        assert price > 100 : "set price over error";
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}
class Order{
    Item item;
    int amount;

    public Item getItem() {
        return item;
    }

    public void setItem(Item item) {
        this.item = item;
    }

    public int getAmount() {
        return amount;
    }

    public void setAmount(int amount) {
        this.amount = amount;
    }

    public Order(Item item, int amount) {
        this.item = item;
        this.amount = amount;
    }
    int calc() {
        return this.item.getPrice() * this.amount;
    }
}
public class Outer {
    public static void main(String args[]) {
        Item i = null;
        i = new Item("apple",50);
        i.setPrice(150);
        int amount = 30;
        Order o = new Order(i,amount);
        int oPrice = o.calc();
        assert oPrice == i.getPrice() * amount: "calc price error";
    }
}

C:\Users\***\Documents\NetBeansProjects\mavenproject1\target\classes>java -ea com.mycompany.mavenproject1.Outer
Exception in thread "main" java.lang.AssertionError: constructor price over error
        at com.mycompany.mavenproject1.Item.<init>(Outer.java:15)
        at com.mycompany.mavenproject1.Outer.main(Outer.java:66)