custom Annotation

NotNull Annotationを作り、NotNullチェックする
annotationで指定したメッセージを表示する
class定義からgetDeclaredFieldsでfield定義を取り出している
f.get(x(instance))でxから値取得する。
これをリフレクションという。

@Retention(RetentionPolicy.RUNTIME)
@interface NotNull{
    String message();
}
class Sample {
    @NotNull(message = " is not null x1")
    private String name;
    @NotNull(message = " is not null x2")
    private String category;
    private String city;

    public Sample(String name, String category, String city) {
        this.name = name;
        this.category = category;
        this.city = city;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

}
public class Outer {
    public static void main(String args[]) {
        Sample s = new Sample(null,null,null);
        method(s);
        s = new Sample("",null,null);
        method(s);
    }
    static void method(Sample s) {
        System.out.println("-------------");
        Field[] fs = s.getClass().getDeclaredFields();
        for(Field f : fs) {
            f.setAccessible(true);
            NotNull n = f.getAnnotation(NotNull.class);
            if(n!=null) {
                String msg = n.message();
                String name = "";
                String fieldName = "";
                try {
                    fieldName = f.getName();
                    name = (String) f.get(s);
                } catch (IllegalAccessException ex ) {
                    ex.printStackTrace();
                }
                if(name == null) {
                    System.out.println(fieldName + msg);
                }
            }
        }
    }
}

-------------
name is not null x1
category is not null x2
-------------
category is not null x2