Field Reflection

A Field provides information about a field of a class or an interface.

Get the field modifiers, name and type

int getModifiers(): Returns the Java language modifiers for the field.
String getName(): Returns the name of the field.
Class<?> getType(): Returns a Class object that identifies the declared type.

Is this field an Enum, is it Synthetic

boolean isEnumConstant(): Returns true if this field represents an element of an enumerated type; returns false otherwise.
boolean isSynthetic(): Returns true if this field is a synthetic field; returns false otherwise.

Get annotation for this field

<T extends Annotation>T getAnnotation(Class<T> annotationClass): Returns this element's annotation.
Annotation[] getDeclaredAnnotations(): Returns all annotations that are directly present on this element.

Get the class for this field

Class<?> getDeclaringClass(): Returns the Class object representing the class or interface that declares the field.

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.

Set field value 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.

Get the string representation of a field

String toGenericString(): Returns a string describing this Field, including its generic type.
String toString(): Returns a string describing this Field.

Revised from Open JDK source code

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

 
import java.lang.reflect.Field;

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);
  }
}


10
20
3.14
Exception in thread "main" java.lang.IllegalAccessException: Can not set static final double field X.PI to (double)20.0
  at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:55)
  at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:91)
  at sun.reflect.UnsafeQualifiedStaticDoubleFieldAccessorImpl.setDouble(UnsafeQualifiedStaticDoubleFieldAccessorImpl.java:141)
  at java.lang.reflect.Field.setDouble(Field.java:889)
  at Main.main(Main.java:18)

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