06s-reflection-annotations.html

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title: Reflection and annotations
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---

# Reflection and Annotations

.center[![reflection](/assets/reflection.gif)]

- `java.lang.Class<T>`: your ticket to reflection
- Messing with Objects
- Basic Java beans (a simple plugin architecture)
- Annotations

---

# Defining Annotations

```java
public @interface MyMarker {}
```

```java
public @interface MyValue {
    String value();
}
```

```java
public @interface Fixed {
    String author() ;
    String date() ;
    String bugsFixed() default "" ;
}
```

```java
@MyMarker
@MyValue("Hansdampf")
@Fixed(author="riko493", date="2017-11-15")
void method() { ... }
```

```java
@MyMarker
@SomeValue("meh")
void method() { ... }
```

---

# Method Annotations

```java
class K {
    @Override
    public boolean equals(Object o) {
        // ...
    }
    @Deprecated
    public void useSomethingElseNow() {
        // ...
    }
    @SuppressWarnings("unchecked")
    public void nastyCasts() {

    }
}
```

---

# Type (Attribute) Annotations

`@NonNull`: The compiler can determine cases where a code path might receive a null value, without ever having to debug a `NullPointerException`.

`@ReadOnly`: The compiler will flag any attempt to change the object.

`@Regex`: Provides compile-time verification that a `String` intended to be used as a regular expression is a properly formatted regular expression.

`@Tainted` and `@Untainted`: Identity types of data that should not be used together, such as remote user input being used in system commands, or sensitive information in log streams.

```java
abstract void method(@NonNull String value, @Regex re);
```

---

# Annotations: JUnit5

```java
class MyTest {
    BufferedReader reader;

    @BeforeAll
    void setUp() {
        reader = new BufferedReader();  // ...
    }

    @Test
    void testSomeClass() {
        // ...
    }
}
```

---

# Annotations: Gson by Google

```java
class Klass {
    private int value1 = 1;
    private String value2 = "abc";
    @SerializedName("odd-name") private String oddName = "1337";
    private transient int value3 = 3;  // will be excluded
    Klass() {
        // default constructor (required)
    }
}

// Serialization
Klass obj = new Klass();
Gson gson = new Gson();
String json = gson.toJson(obj);  
// ==> json is {"value1":1,"value2":"abc","odd-name": "1337"}

// Deserialization
Klass obj2 = gson.fromJson(json, Klass.class);
// ==> obj2 is just like obj
```

---

# Annotations: Butterknife...

Don't do this...

```java
class ExampleActivity {
    TextView title;
    TextView subtitle;
    TextView footer;
public View onCreate(Bundle savedInstanceState) {
   // [...]
   View view = inflater.inflate(R.layout.myLayout, container, false);

   title = (ImageView) view.findViewById(R.id.title);
   subtitle = (ImageView) view.findViewById(R.id.subtitle);
   footer = (ImageView) view.findViewById(R.id.footer);
}
```

---

# Annotations: Butterknife

```java
class ExampleActivity extends Activity {
    @BindView(R.id.title) TextView title;
    @BindView(R.id.subtitle) TextView subtitle;
    @BindView(R.id.footer) TextView footer;

    @Override public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.simple_activity);
        ButterKnife.bind(this);  // that's it, all done!
    }
}
```

---

# Annotations: Retrofit

```java
public interface GitHubService {
  @GET("users/{user}/repos")
  Call<List<Repo>> listRepos(@Path("user") String user);
}

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.github.com/")
    .build();

GitHubService service = retrofit.create(GitHubService.class);

Call<List<Repo>> repos = service.listRepos("octocat");
```

---

# Annotations: Lombok

> Java is so lame, it doesn't even support properties.

```java
class SoMuchBoilerplate {
    private String hidden;

    // simple constructor
    SoMuchBoilerplate(String h) {
        hidden = h;
    }

    // public getter
    public int getHidden() {
        return hidden;
    }
    
    // package setter
    void setHidden(String h) { 
        hidden = h;
    }

    // basic toString for debug
    public String toString() {
        return "SoMuchBoilerplate(" + "hidden=" + hidden + ")");
    }
}
```

---

# Annotations: Lombok

Fear not, [Lombok](https://projectlombok.org/)'s annotation processor has you covered.

```java
@Data
SoMuchBoilerplate {
    @NonNull @Setter(AccessLevel.PACKAGE)
    private int hidden;
}
```