Java : Field (reflection) with Examples

public class AccessTest {
    private int num = 100;
}

final var field = AccessTest.class.getDeclaredField("num");
System.out.println(field); // private int AccessTest.num

final var obj = new AccessTest();

System.out.println(field.canAccess(obj)); // false
//field.getInt(obj); // IllegalAccessException

System.out.println(field.trySetAccessible()); // true
System.out.println(field.canAccess(obj)); // true

System.out.println(field.getInt(obj)); // 100

DECLARED, PUBLIC

Please see the link below.

• Java API Examples : Member

boolean equals (Object obj)

Compares this Field against the specified object.

class A {
    int num;
    String str;
}

final var a1 = new A();
final var a2 = new A();

final var num1 = a1.getClass().getDeclaredField("num");
final var num2 = a2.getClass().getDeclaredField("num");

System.out.println(num1.equals(num2)); // true

final var str = a1.getClass().getDeclaredField("str");

System.out.println(str.equals(num1)); // false

Object get (Object obj)

Returns the value of the field represented by this Field, on the specified object.

public class GetTest {
    public int num1 = 100;
    public String str1 = "abcd";

    public static int num2 = 999;
    private String str2 = "XYZ";
}

final var cls = GetTest.class;
final var obj = new GetTest();

final var num1 = cls.getDeclaredField("num1");
System.out.println("num1 : " + num1.get(obj));

final var str1 = cls.getDeclaredField("str1");
System.out.println("str1 : " + str1.get(obj));

final var num2 = cls.getDeclaredField("num2");
System.out.println("num2 : " + num2.get(null));

try {
    final var str2 = cls.getDeclaredField("str2");
    final var ret = str2.get(obj);

} catch (IllegalAccessException e) {
    System.out.println("str2 : IllegalAccessException!");
}

// Result
// ↓
//num1 : 100
//str1 : abcd
//num2 : 999
//str2 : IllegalAccessException!

AnnotatedType getAnnotatedType ()

Returns an AnnotatedType object that represents the use of a type to specify the declared type of the field represented by this Field.

public class Foo {
    public int num;
    public String str;
}

final var cls = Foo.class;

final var numField = cls.getDeclaredField("num");
System.out.println(numField); // public int Foo.num

final var numType = numField.getAnnotatedType();
System.out.println(numType); // int

final var strField = cls.getDeclaredField("str");
System.out.println(strField); // public java.lang.String Foo.str

final var strType = strField.getAnnotatedType();
System.out.println(strType); // java.lang.String

<T extends Annotation> T getAnnotation (Class<T> annotationClass)

Returns this element's annotation for the specified type if such an annotation is present, else null.

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Foo {
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Bar {
}

public class A {
    @Foo
    public int num;

    @Bar
    public String str;
}

final var field = A.class.getDeclaredField("num");
System.out.println(field); // public int A.num

final var ret1 = field.getAnnotation(Foo.class);
System.out.println(ret1); // @Foo()

final var ret2 = field.getAnnotation(Bar.class);
System.out.println(ret2); // null

final var field = A.class.getDeclaredField("str");
System.out.println(field); // public java.lang.String A.str

final var ret1 = field.getAnnotation(Foo.class);
System.out.println(ret1); // null

final var ret2 = field.getAnnotation(Bar.class);
System.out.println(ret2); // @Bar()

<T extends Annotation> T[] getAnnotationsByType (Class<T> annotationClass)

Returns annotations that are associated with this element.

@Repeatable(FooArray.class)
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Foo {
    String value();
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface FooArray {
    Foo[] value();
}

public class A {
    @Foo("a1")
    @Foo("a2")
    public int num;
}

final var field = A.class.getDeclaredField("num");
System.out.println(field);

System.out.println("-- annotations --");
for (final var a : field.getAnnotationsByType(Foo.class)) {
    System.out.println(a.annotationType().getSimpleName() + " : " + a.value());
}

// Result
// ↓
//public int A.num
//-- annotations --
//Foo : a1
//Foo : a2

boolean getBoolean (Object obj)

Gets the value of a static or instance boolean field.

Please see also : get(Object obj)

public class Foo {
    public boolean value = true;
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public boolean Foo.value

    final var ret = field.getBoolean(obj);
    System.out.println(ret); // true
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        final var ret = field.getBoolean(obj);
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Attempt to get java.lang.String field "Foo.str" with illegal data type
    //  conversion to boolean
}

byte getByte (Object obj)

Gets the value of a static or instance byte field.

This method is equivalent except a type to getInt(Object obj).

char getChar (Object obj)

Gets the value of a static or instance field of type char or of another primitive type convertible to type char via a widening conversion.

Please see also : get(Object obj)

public class Foo {
    public char value = 'a';
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public char Foo.value

    final var ret = field.getChar(obj);
    System.out.println(ret); // a
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        final var ret = field.getChar(obj);
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Attempt to get java.lang.String field "Foo.str" with illegal data type
    //  conversion to char
}

Class<?> getDeclaringClass ()

Returns the Class object representing the class or interface that declares the field represented by this Field object.

public class AccessTest {
    private int num = 100;
}

final var cls = AccessTest.class;

final var field = cls.getDeclaredField("num");
System.out.println(field); // private int AccessTest.num

final var ret = field.getDeclaringClass();
System.out.println(ret); // class AccessTest
System.out.println(cls.equals(ret)); // true

double getDouble (Object obj)

Gets the value of a static or instance field of type double or of another primitive type convertible to type double via a widening conversion.

Please see also : get(Object obj)

public class Foo {
    public double value = 1.23;
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public double Foo.value

    final var ret = field.getDouble(obj);
    System.out.println(ret); // 1.23
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        final var ret = field.getDouble(obj);
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Attempt to get java.lang.String field "Foo.str" with illegal data type
    //  conversion to double
}

float getFloat (Object obj)

Gets the value of a static or instance field of type float or of another primitive type convertible to type float via a widening conversion.

This method is equivalent except a type to getDouble(Object obj).

Type getGenericType ()

Returns a Type object that represents the declared type for the field represented by this Field object.

public class Foo<T> {
    public T value;
    public int num;
}

final var foo = new Foo<String>();

final var field = foo.getClass().getDeclaredField("value");
System.out.println(field); // public java.lang.Object Foo.value

final var ret1 = field.getGenericType();
System.out.println(ret1); // T

final var ret2 = field.getType();
System.out.println(ret2); // class java.lang.Object

final var field = Foo.class.getDeclaredField("num");
System.out.println(field); // public int Foo.num

final var ret1 = field.getGenericType();
System.out.println(ret1); // int

final var ret2 = field.getType();
System.out.println(ret2); // int

int getInt (Object obj)

Gets the value of a static or instance field of type int or of another primitive type convertible to type int via a widening conversion.

Please see also : get(Object obj)

public class Foo {
    public int value = 123;
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public int Foo.value

    final var ret = field.getInt(obj);
    System.out.println(ret); // 123
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        final var ret = field.getInt(obj);
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Attempt to get java.lang.String field "Foo.str" with illegal data type
    //  conversion to int
}

long getLong (Object obj)

Gets the value of a static or instance field of type long or of another primitive type convertible to type long via a widening conversion.

This method is equivalent except a type to getInt(Object obj).

int getModifiers ()

Returns the Java language modifiers for the field represented by this Field object, as an integer.

class Foo {
    public int a;
    protected int b;
    private int c;
}

final var cls = Foo.class;

for (final var field : cls.getDeclaredFields()) {
    final var mod = field.getModifiers();
    System.out.println("name = " + field.getName() + " : mod = " + mod);
    System.out.println("  isPublic : " + Modifier.isPublic(mod));
    System.out.println("  isProtected : " + Modifier.isProtected(mod));
    System.out.println("  isPrivate : " + Modifier.isPrivate(mod));
}

// Result
// ↓
//name = a : mod = 1
//  isPublic : true
//  isProtected : false
//  isPrivate : false
//name = b : mod = 4
//  isPublic : false
//  isProtected : true
//  isPrivate : false
//name = c : mod = 2
//  isPublic : false
//  isProtected : false
//  isPrivate : true

String getName ()

Returns the name of the field represented by this Field object.

public class Foo {
    public int num;
    public String str;
}

final var cls = Foo.class;

final var num = cls.getDeclaredField("num");
System.out.println(num); // public int Foo.num
System.out.println(num.getName()); // num

final var str = cls.getDeclaredField("str");
System.out.println(str); // public java.lang.String Foo.str
System.out.println(str.getName()); // str

short getShort (Object obj)

Gets the value of a static or instance field of type short or of another primitive type convertible to type short via a widening conversion.

This method is equivalent except a type to getInt(Object obj).

Class<?> getType ()

Returns a Class object that identifies the declared type for the field represented by this Field object.

Please see getGenericType().

int hashCode ()

Returns a hashcode for this Field.

class Foo {
    int num;
    String str;
}

final var cls = Foo.class;

final var ret1 = cls.getDeclaredField("num").hashCode();
System.out.println(ret1); // -1850314582

final var ret2 = cls.getDeclaredField("str").hashCode();
System.out.println(ret2); // -1850317539

boolean isEnumConstant ()

Returns true if this field represents an element of an enumerated class; returns false otherwise.

enum Foo {
    E1,
    E2,
}

class Bar {
    int num;
}

final var e1 = Foo.class.getDeclaredField("E1");
System.out.println(e1.isEnumConstant()); // true

final var num = Bar.class.getDeclaredField("num");
System.out.println(num.isEnumConstant()); // false

boolean isSynthetic ()

Returns true if this field is a synthetic field; returns false otherwise.

class Foo {
    int num;
}

final var field = Foo.class.getDeclaredField("num");
System.out.println(field.isSynthetic()); // false

void set (Object obj, Object value)

Sets the field represented by this Field object on the specified object argument to the specified new value.

public class SetTest {
    public int num1 = 100;
    public String str1 = "abcd";

    public static int num2 = 999;
    private String str2 = "XYZ";
}

final var cls = SetTest.class;
final var obj = new SetTest();

final var field = cls.getDeclaredField("num1");
System.out.println(field); // public int SetTest.num1

System.out.println(field.getInt(obj)); // 100

field.set(obj, 456);

System.out.println(field.getInt(obj)); // 456

final var cls = SetTest.class;
final var obj = new SetTest();

final var field = cls.getDeclaredField("str1");
System.out.println(field); // public java.lang.String SetTest.str1

System.out.println(field.get(obj)); // abcd

field.set(obj, "defg");

System.out.println(field.get(obj)); // defg

final var cls = SetTest.class;

final var field = cls.getDeclaredField("num2");
System.out.println(field); // public static int SetTest.num2

System.out.println(field.getInt(null)); // 999

field.set(null, 1000);

System.out.println(field.getInt(null)); // 1000

final var cls = SetTest.class;
final var obj = new SetTest();

final var field = cls.getDeclaredField("str2");
System.out.println(field); // private java.lang.String SetTest.str2

System.out.println(field.canAccess(obj)); // false

//field.set(obj, "AAA"); // IllegalAccessException

System.out.println(field.trySetAccessible()); // true
System.out.println(field.canAccess(obj)); // true

System.out.println(field.get(obj)); // XYZ

field.set(obj, "AAA");

System.out.println(field.get(obj)); // AAA

void setAccessible (boolean flag)

Set the accessible flag for this reflected object to the indicated boolean value.

public class AccessTest {
    private int num = 100;
}

final var field = AccessTest.class.getDeclaredField("num");
final var obj = new AccessTest();

System.out.println(field.canAccess(obj)); // false

field.setAccessible(true);
System.out.println(field.canAccess(obj)); // true

field.setAccessible(false);
System.out.println(field.canAccess(obj)); // false

void setBoolean (Object obj, boolean z)

Sets the value of a field as a boolean on the specified object.

Please see also : set(Object obj, Object value)

public class Foo {
    public boolean value = true;
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public boolean Foo.value

    System.out.println(field.getBoolean(obj)); // true

    field.setBoolean(obj, false);

    System.out.println(field.getBoolean(obj)); // false
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        field.setBoolean(obj, false);
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Can not set java.lang.String field Foo.str to (boolean)false
}

void setByte (Object obj, byte b)

Sets the value of a field as a byte on the specified object.

This method is equivalent except a type to setInt(Object obj, int i).

void setChar (Object obj, char c)

Sets the value of a field as a char on the specified object.

Please see also : set(Object obj, Object value)

public class Foo {
    public char value = 'a';
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public char Foo.value

    System.out.println(field.getChar(obj)); // a

    field.setChar(obj, 'b');

    System.out.println(field.getChar(obj)); // b
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        field.setChar(obj, 'b');
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Can not set java.lang.String field Foo.str to (char)b
}

void setDouble (Object obj, double d)

Sets the value of a field as a double on the specified object.

Please see also : set(Object obj, Object value)

public class Foo {
    public double value = 1.23;
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public double Foo.value

    System.out.println(field.getDouble(obj)); // 1.23

    field.setDouble(obj, 4.56);

    System.out.println(field.getDouble(obj)); // 4.56
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        field.setDouble(obj, 4.56);
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Can not set java.lang.String field Foo.str to (double)4.56
}

void setFloat (Object obj, float f)

Sets the value of a field as a float on the specified object.

This method is equivalent except a type to setDouble(Object obj, double d).

void setInt (Object obj, int i)

Sets the value of a field as an int on the specified object.

Please see also : set(Object obj, Object value)

public class Foo {
    public int value = 123;
    public String str = "abcd";
}

final var cls = Foo.class;
final var obj = new Foo();

{
    final var field = cls.getDeclaredField("value");
    System.out.println(field); // public int Foo.value

    System.out.println(field.getInt(obj)); // 123

    field.setInt(obj, 456);

    System.out.println(field.getInt(obj)); // 456
}
{
    final var field = cls.getDeclaredField("str");
    System.out.println(field); // public java.lang.String Foo.str

    try {
        field.setInt(obj, 456);
    } catch (IllegalArgumentException e) {
        System.out.println(e);
    }

    // Result
    // ↓
    //java.lang.IllegalArgumentException:
    //  Can not set java.lang.String field Foo.str to (int)456
}

void setLong (Object obj, long l)

Sets the value of a field as a long on the specified object.

This method is equivalent except a type to setInt(Object obj, int i).

void setShort (Object obj, short s)

Sets the value of a field as a short on the specified object.

This method is equivalent except a type to setInt(Object obj, int i).

String toGenericString ()

Returns a string describing this Field, including its generic type.

public class Foo<T> {
    public T value;
    public int num;
}

final var foo = new Foo<String>();

final var field = foo.getClass().getDeclaredField("value");

final var ret1 = field.toGenericString();
System.out.println(ret1); // public T Foo.value

final var ret2 = field.toString();
System.out.println(ret2); // public java.lang.Object Foo.value

final var field = Foo.class.getDeclaredField("num");

final var ret1 = field.toGenericString();
System.out.println(ret1); // public int Foo.num

final var ret2 = field.toString();
System.out.println(ret2); // public int Foo.num

String toString ()

Returns a string describing this Field.

Please see toGenericString().

canAccess, getAnnotations, getDeclaredAnnotation, getDeclaredAnnotations, getDeclaredAnnotationsByType, isAccessible, isAnnotationPresent, setAccessible, trySetAccessible

Please see the link below.

• Java API Examples : AccessibleObject