Get/Set field value

In this chapter you will learn:

1. How to get value from a field
2. How to set field value with Reflection
3. Value for a public final static field cannot be set through Java reflection

Get value from a field

We can get field value with reflection methods and they returns primitive types. The following lists have all methods we need to know.

• Object get(Object obj) Returns field value
• boolean getBoolean(Object obj) Gets field value as boolean.
• byte getByte(Object obj) Gets field value as byte.
• char getChar(Object obj) Gets field value as char.
• double getDouble(Object obj) Gets field value as double.
• float getFloat(Object obj) Gets field value as float.
• Type getGenericType() Returns a Type object that represents the declared type for the field represented by this Field object.
• int getInt(Object obj) Gets field value as int.
• long getLong(Object obj) Gets field value as long.
• short getShort(Object obj) Gets field value as short.

import java.awt.Rectangle;
import java.lang.reflect.Field;
/*java 2 s .co  m*/
public class Main {

  public static void main(String[] args) {
    Rectangle r = new Rectangle(100, 325);
    Class c = r.getClass();
    try {
      Field heightField = c.getField("height");
      Integer heightValue = (Integer) heightField.get(r);
      System.out.println("Height: " + heightValue.toString());
    } catch (NoSuchFieldException e) {
      System.out.println(e);
    } catch (SecurityException e) {
      System.out.println(e);
    } catch (IllegalAccessException e) {
      System.out.println(e);
    }
  }
}

The output:

Set field value with Reflection

We can also set the value back to a field with reflection.

• void set(Object obj, Object value) Sets the field value.
• void setBoolean(Object obj, boolean z) Sets boolean field value.
• void setByte(Object obj, byte b) Sets byte field value.
• void setChar(Object obj, char c) Sets char field value.
• void setDouble(Object obj, double d) Sets double field value.
• void setFloat(Object obj, float f) Sets float field value.
• void setInt(Object obj, int i) Sets int field value.
• void setLong(Object obj, long l) Sets long field value.
• void setShort(Object obj, short s) Sets short field value.

import java.awt.Rectangle;
import java.lang.reflect.Field;
/* ja  v a  2s. c o m*/
public class Main {

  public static void main(String[] args) {
    Rectangle r = new Rectangle(100, 325);
    Class c = r.getClass();
    try {
      Field heightField = c.getField("height");
      heightField.setInt(r, 1000);
      Integer heightValue = (Integer) heightField.get(r);
      System.out.println("Height: " + heightValue.toString());
    } catch (NoSuchFieldException e) {
      System.out.println(e);
    } catch (SecurityException e) {
      System.out.println(e);
    } catch (IllegalAccessException e) {
      System.out.println(e);
    }
  }
}

Get/set field value and possible exceptions

The following code gets and sets the values of instance and class fields

import java.lang.reflect.Field;
/*from ja v  a 2  s. c om*/
class X {
  public int i = 10;
  public static final double PI = 3.14;
}

public class Main {
  public static void main(String[] args) throws Exception {
    Class<?> clazz = Class.forName("X");
    X x = (X) clazz.newInstance();
    Field f = clazz.getField("i");
    System.out.println(f.getInt(x)); // Output: 10
    f.setInt(x, 20);
    System.out.println(f.getInt(x)); // Output: 20
    f = clazz.getField("PI");
    System.out.println(f.getDouble(null)); // Output: 3.14
    f.setDouble(x, 20);
  }
}

Here is the output from the code above. The exception is for the final static field PI. public final static field defines a constant in Java and we cannot reset a value for a constant through reflection.

Next chapter...

What you will learn in the next chapter:

1. Using java.lang.reflect.Method to do method reflection
2. Find out Method's return type, modifiers and parameters
3. How to invoke a method

Reflection

Class Reflection

Class modifier, package and string presentation

Constructor reflection

Field Reflection

Get/Set field value

Java method reflection

Modifier

Package

Array reflection

Get annotation type

Method annotation