Trail: The Reflection API
Lesson: Members
Section: Constructors
Finding Constructors
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Finding Constructors

A constructor declaration includes the name, modifiers, parameters, and list of throwable exceptions. The java.lang.reflect.Constructor class provides a way to obtain this information.

The ConstructorSift example illustrates how to search a class's declared constructors for one which has a parameter of a given type.

import java.lang.reflect.Constructor;
import java.lang.reflect.Type;
import static java.lang.System.out;

public class ConstructorSift {
    public static void main(String... args) {
	try {
	    Class<?> cArg = Class.forName(args[1]);

	    Class<?> c = Class.forName(args[0]);
	    Constructor[] allConstructors = c.getDeclaredConstructors();
	    for (Constructor ctor : allConstructors) {
		Class<?>[] pType  = ctor.getParameterTypes();
		for (int i = 0; i < pType.length; i++) {
		    if (pType[i].equals(cArg)) {
			out.format("%s%n", ctor.toGenericString());

			Type[] gpType = ctor.getGenericParameterTypes();
			for (int j = 0; j < gpType.length; j++) {
			    char ch = (pType[j].equals(cArg) ? '*' : ' ');
			    out.format("%7c%s[%d]: %s%n", ch,
				       "GenericParameterType", j, gpType[j]);

        // production code should handle this exception more gracefully
	} catch (ClassNotFoundException x) {

Method.getGenericParameterTypes() will consult the Signature Attribute in the class file if it's present. If the attribute isn't available, it falls back on Method.getParameterType() which was not changed by the introduction of generics. The other methods with name getGenericFoo() for some value of Foo in reflection are implemented similarly. The syntax for the returned values of Method.get*Types() is described in Class.getName().

Here is the output for all constructors in java.util.Formatter which have a Locale argument.

$ java ConstructorSift java.util.Formatter java.util.Locale
       GenericParameterType[0]: class
       GenericParameterType[1]: class java.lang.String
      *GenericParameterType[2]: class java.util.Locale
public java.util.Formatter(java.lang.String,java.lang.String,java.util.Locale)
       GenericParameterType[0]: class java.lang.String
       GenericParameterType[1]: class java.lang.String
      *GenericParameterType[2]: class java.util.Locale
public java.util.Formatter(java.lang.Appendable,java.util.Locale)
       GenericParameterType[0]: interface java.lang.Appendable
      *GenericParameterType[1]: class java.util.Locale
public java.util.Formatter(java.util.Locale)
      *GenericParameterType[0]: class java.util.Locale
public java.util.Formatter(,java.lang.String,java.util.Locale)
       GenericParameterType[0]: class
       GenericParameterType[1]: class java.lang.String
      *GenericParameterType[2]: class java.util.Locale

The next example output illustrates how to search for a parameter of type char[] in String.

$ java ConstructorSift java.lang.String "[C"
       GenericParameterType[0]: int
       GenericParameterType[1]: int
      *GenericParameterType[2]: class [C
public java.lang.String(char[],int,int)
      *GenericParameterType[0]: class [C
       GenericParameterType[1]: int
       GenericParameterType[2]: int
public java.lang.String(char[])
      *GenericParameterType[0]: class [C

The syntax for expressing arrays of reference and primitive types acceptable to Class.forName() is described in Class.getName(). Note that the first listed constructor is package-private, not public. It is returned because the example code uses Class.getDeclaredConstructors() rather than Class.getConstructors(), which returns only public constructors.

This example shows that searching for arguments of variable arity (which have a variable number of parameters) requires use of array syntax:

$ java ConstructorSift java.lang.ProcessBuilder "[Ljava.lang.String;"
public java.lang.ProcessBuilder(java.lang.String[])
      *GenericParameterType[0]: class [Ljava.lang.String;

This is the actual declaration of the ProcessBuilder constructor in source code:

public ProcessBuilder(String... command)

The parameter is represented as a single-dimension array of type java.lang.String. This can be distinguished from a parameter that is explicitly an array of java.lang.String by invoking Constructor.isVarArgs().

The final example reports the output for a constructor which has been declared with a generic parameter type:

$ java ConstructorSift java.util.HashMap java.util.Map
public java.util.HashMap(java.util.Map<? extends K, ? extends V>)
      *GenericParameterType[0]: java.util.Map<? extends K, ? extends V>

Exception types may be retrieved for constructors in a similar way as for methods. See the MethodSpy example described in Obtaining Method Type Information section for further details.

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