I was given a class (which I could not change) with a lot of fields of String, each of them have a set method.
public class C {
private String fieldA,
private String fieldB,
...
public void setFieldA(String s) {
this.fieldA = s;
}
public void setFieldB(String s) {
this.fieldB = s;
}
...
}
I need to write code to create an instance of the class and initialize it with a json object (which will have value for some or all of the fields).
The json is very simple and looks like:
{"fieldA":"valueA", "fieldB":"valueB"..... }
I know I could write something like below for every field:
if (myJsonObj.containsKey("fieldA"){
myInstance.setFieldA(myJsonObj.get("fieldA").toString());
}
I'm wondering if there is a smarter way I could do this, since the number of fields could be up to about thirty. Instead of repeating the above code thirty times (and the only thing different between each is "fieldA" in the above code).
Thanks!
Sounds like a job for reflection to me. Everything except the bit marked with "Reflection" is here solely to make this an SSCCE. You'll likely have your own way to track which methods are available or whatever. Take a look at the following site for an (IMHO) excellent guide (I am not affiliated with the site in any way): Guide to Java Reflection
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class Test {
private String fieldA;
private String fieldB;
private String fieldC;
private String fieldD;
private String fieldE;
public String getFieldA() {
return fieldA;
}
public void setFieldA(final String fieldA) {
this.fieldA = fieldA;
}
public String getFieldB() {
return fieldB;
}
public void setFieldB(final String fieldB) {
this.fieldB = fieldB;
}
public String getFieldC() {
return fieldC;
}
public void setFieldC(final String fieldC) {
this.fieldC = fieldC;
}
public String getFieldD() {
return fieldD;
}
public void setFieldD(final String fieldD) {
this.fieldD = fieldD;
}
public String getFieldE() {
return fieldE;
}
public void setFieldE(final String fieldE) {
this.fieldE = fieldE;
}
private static final char[] chars = {'A', 'B', 'C', 'D', 'E'};
private static final String[] values =
{"valueA", "valueB", "valueC", "valueD", "valueE"};
private static final Class<?>[] types = {String.class};
public static void main(final String[] args)
throws SecurityException, NoSuchMethodException, IllegalArgumentException,
IllegalAccessException, InvocationTargetException {
// Reflection setting values - everything else is here solely to make this an SSCCE
final Test test = new Test();
final Class<Test> clazz = Test.class;
for(int i = 0; i < chars.length; i++) {
final String name = "setField" + chars[i];
final Method method = clazz.getDeclaredMethod(name, types);
final Object[] params = {values[i]};
method.invoke(test, params);
}
// End of reflection
System.out.println(test.getFieldA());
System.out.println(test.getFieldB());
System.out.println(test.getFieldC());
System.out.println(test.getFieldD());
System.out.println(test.getFieldE());
}
}
Thanks to Chris Parker.
Here is my final code based on his original code. Because there are many String fields in the class and it might change in the future, so using reflect makes the update/change very easy: I only need to change the String Array.
The given class is like below (only String fields and their set methods are shown):
public class MyClass {
private String fieldA,
private String fieldB,
...
public void setFieldA(String s) {
this.fieldA = s;
}
public void setFieldB(String s) {
this.fieldB = s;
}
...
}
The method to set the value of those string fields using a JSONObject input to an instance of C:
public void fillClassStringFields(MyClass c, JSONObject input) {
String[] names = {"FieldA","FieldB",...};
Class<MyClass> clazz = MyClass.class;
Class<?>[] types = {String.class};
for(int i = 0; i< names.length;i++) {
String name = names[i].toString();
String methodName = "set" + name;
if(input.containsKey(name)) {
try {
Method method = clazz.getDeclaredMethod(methodName, types);
method.invoke(c, input.get(name).toString());
} catch (NoSuchMethodException | SecurityException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
}
}
I'm trying to access a private method, but it doesn't let me to, and throws the exception. How can I avoid the exception? I've tried a lot of other questions, but they all just point to using setAccessible(true), which I'm already using right now.
import java.lang.reflect.*;
class Reflection {
public static void main (String[] args) {
try {
//Get class instance
ReflectionExamples re = new ReflectionExamples();
Class<?> c = re.getClass();
System.out.println("Class name is " + c.getName());
//Get constructor of class
Constructor<?> cons = c.getConstructor();
System.out.println("Constructor name is " + cons.getName());
System.out.println("List of methods are:");
//Iterate all methods
Method[] methods = c.getMethods();
for (Method method : methods) {
System.out.println(method.getName());
}
//Desired method to call
Method call = c.getDeclaredMethod("e");
//Unlock the private Method
call.setAccessible(true);
//Invoke the Method
call.invoke(re);
//Desired method to call
Method call2 = c.getDeclaredMethod("a");
//Invoke the method
call2.invoke(c);
} catch(Throwable e) {
System.err.println(e);
}
}
}
class ReflectionExamples {
private String string = "String222222";
public int i = 0;
public ReflectionExamples() {
string = "String";
}
private void e() {
System.out.println("Char entered is not valid.");
}
public void f(char a) {
System.out.println("awojiewfihuerhkuah");
}
public void a() {
System.out.println(string);
}
}
And I get:
java.security.AccessControlException: access denied ("java.lang.reflect.ReflectPermission" "suppressAccessChecks")
public class Test {
public static void main(String[] args) throws Exception {
try {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int num = Integer.parseInt(br.readLine().trim());
Object o;
Method[] methods = Inner.class.getEnclosingClass().getMethods();
for(int i=0;i<methods.length;i++) {
System.out.println(methods[i].invoke(new Solution(),8));
}
// Call powerof2 method here
} catch (Exception e) {
e.printStackTrace();
}
}
static class Inner {
private class Private {
private String powerof2(int num) {
return ((num & num - 1) == 0) ? "power of 2" : "not a power of 2";
}
}
}
}
Is it possible to call powerof2() method ?
I am getting java.lang.IllegalArgumentException: argument type mismatch for invoke
Yes, things declared in the same top-level class can always access each other:
public class Test {
public static void main(String[] args) throws Exception {
Inner i = new Inner(); // Create an instance of Inner
Inner.Private p = i.new Private(); // Create an instance of Private through
// the instance of Inner, this is needed since
// Private is not a static class.
System.out.println(p.powerof2(2)); // Call the method
}
static class Inner {
private class Private {
private String powerof2(int num) {
return ((num & num - 1) == 0) ? "power of 2" : "not a power of 2";
}
}
}
}
See Ideone
Reflection version:
public class Test {
public static void main(String[] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, SecurityException, NoSuchMethodException {
Class<?> privateCls = Inner.class.getDeclaredClasses()[0];
Method powerMethod = privateCls.getDeclaredMethod("powerof2", int.class);
powerMethod.setAccessible(true);
Constructor<?> constructor = privateCls.getDeclaredConstructors()[0];
constructor.setAccessible(true);
Object instance = constructor.newInstance(new Inner());
System.out.println(powerMethod.invoke(instance, 2));
}
static class Inner {
private class Private {
private String powerof2(int num) {
return ((num & num - 1) == 0) ? "power of 2" : "not a power of 2";
}
}
}
}
Accessing the FileType values in converterfactory. I have a class named FileType:
public final class FileType
{
public static final FileType VALUES[];
public static final FileType WORD;
static
{
WORD= new FileType("WORD", 0);
VALUES = (new FileType[] { WORD});
}
public static final FileType[] values()
{
return (FileType[])VALUES.clone();
}
String s;
int i;
private FileType(String msofficedoC, int i) {
this.s= msofficedoC;
this.i=i;
}
}
I have a another class ConverterFactory in which I want to access the number of MSOFFICEDOC as 0 so I have _cls1 class.
static class cls1
{
static final int SwitchMapfile2xliff4jFileType[];
static
{
SwitchMapfiletoxliffFileType = new int[FileType.values().length];
try
{
SwitchMapfiletoxliffFileType[FileType.WORD.ordinal()] = 1;
}
catch(NoSuchFieldError nosuchfielderror) { }
}
}
In FileType.WORD.ordinal() it is giving an error but I want to access FileType.WORD value in switch case
switch(cls1.SwitchMapfiletoxliffFileType[filetype.i])
{
case 1:
}
It is giving an exception
Exception in thread "AWT-EventQueue-0"
java.lang.ExceptionInInitializerError
please give me some suggestion this .
Thank you in advanced
I often use this design in my code to maintain configurable values. Consider this code:
public enum Options {
REGEX_STRING("Some Regex"),
REGEX_PATTERN(Pattern.compile(REGEX_STRING.getString()), false),
THREAD_COUNT(2),
OPTIONS_PATH("options.config", false),
DEBUG(true),
ALWAYS_SAVE_OPTIONS(true),
THREAD_WAIT_MILLIS(1000);
Object value;
boolean saveValue = true;
private Options(Object value) {
this.value = value;
}
private Options(Object value, boolean saveValue) {
this.value = value;
this.saveValue = saveValue;
}
public void setValue(Object value) {
this.value = value;
}
public Object getValue() {
return value;
}
public String getString() {
return value.toString();
}
public boolean getBoolean() {
Boolean booleanValue = (value instanceof Boolean) ? (Boolean) value : null;
if (value == null) {
try {
booleanValue = Boolean.valueOf(value.toString());
}
catch (Throwable t) {
}
}
// We want a NullPointerException here
return booleanValue.booleanValue();
}
public int getInteger() {
Integer integerValue = (value instanceof Number) ? ((Number) value).intValue() : null;
if (integerValue == null) {
try {
integerValue = Integer.valueOf(value.toString());
}
catch (Throwable t) {
}
}
return integerValue.intValue();
}
public float getFloat() {
Float floatValue = (value instanceof Number) ? ((Number) value).floatValue() : null;
if (floatValue == null) {
try {
floatValue = Float.valueOf(value.toString());
}
catch (Throwable t) {
}
}
return floatValue.floatValue();
}
public static void saveToFile(String path) throws IOException {
FileWriter fw = new FileWriter(path);
Properties properties = new Properties();
for (Options option : Options.values()) {
if (option.saveValue) {
properties.setProperty(option.name(), option.getString());
}
}
if (DEBUG.getBoolean()) {
properties.list(System.out);
}
properties.store(fw, null);
}
public static void loadFromFile(String path) throws IOException {
FileReader fr = new FileReader(path);
Properties properties = new Properties();
properties.load(fr);
if (DEBUG.getBoolean()) {
properties.list(System.out);
}
Object value = null;
for (Options option : Options.values()) {
if (option.saveValue) {
Class<?> clazz = option.value.getClass();
try {
if (String.class.equals(clazz)) {
value = properties.getProperty(option.name());
}
else {
value = clazz.getConstructor(String.class).newInstance(properties.getProperty(option.name()));
}
}
catch (NoSuchMethodException ex) {
Debug.log(ex);
}
catch (InstantiationException ex) {
Debug.log(ex);
}
catch (IllegalAccessException ex) {
Debug.log(ex);
}
catch (IllegalArgumentException ex) {
Debug.log(ex);
}
catch (InvocationTargetException ex) {
Debug.log(ex);
}
if (value != null) {
option.setValue(value);
}
}
}
}
}
This way, I can save and retrieve values from files easily. The problem is that I don't want to repeat this code everywhere. Like as we know, enums can't be extended; so wherever I use this, I have to put all these methods there. I want only to declare the values and that if they should be persisted. No method definitions each time; any ideas?
Using an enum to hold configurable values like this looks like an entirely wrong design. Enums are singletons, so effectively you can only have one configuration active at any given time.
An EnumMap sounds more like what you need. It's external to the enum, so you can instantiate as many configurations as you need.
import java.util.*;
public class EnumMapExample {
static enum Options {
DEBUG, ALWAYS_SAVE, THREAD_COUNT;
}
public static void main(String[] args) {
Map<Options,Object> normalConfig = new EnumMap<Options,Object>(Options.class);
normalConfig.put(Options.DEBUG, false);
normalConfig.put(Options.THREAD_COUNT, 3);
System.out.println(normalConfig);
// prints "{DEBUG=false, THREAD_COUNT=3}"
Map<Options,Object> debugConfig = new EnumMap<Options,Object>(Options.class);
debugConfig.put(Options.DEBUG, true);
debugConfig.put(Options.THREAD_COUNT, 666);
System.out.println(debugConfig);
// prints "{DEBUG=true, THREAD_COUNT=666}"
}
}
API links
java.util.EnumMap
A specialized Map implementation for use with enum type keys. All of the keys in an enum map must come from a single enum type that is specified, explicitly or implicitly, when the map is created. Enum maps are represented internally as arrays. This representation is extremely compact and efficient.
i tried doing something similar with enum maps and properties files (please see code below). but my enums were simple and only had one value except for an embedded case. i may have something that is more type safe. i will look around for it.
package p;
import java.util.*;
import java.io.*;
public class GenericAttributes<T extends Enum<T>> {
public GenericAttributes(final Class<T> keyType) {
map = new EnumMap<T, Object>(this.keyType = keyType);
}
public GenericAttributes(final Class<T> keyType, final Properties properties) {
this(keyType);
addStringProperties(properties);
}
public Object get(final T key) {
// what does a null value mean?
// depends on P's semantics
return map.containsKey(key) ? map.get(key) : null;
}
public boolean contains(final T key) {
return map.containsKey(key);
}
public void change(final T key, final Object value) {
remove(key);
put(key, value);
}
public Object put(final T key, final Object value) {
if (map.containsKey(key))
throw new RuntimeException("map already contains: " + key);
else
return map.put(key, value);
}
public Object remove(final T key) {
if (!map.containsKey(key))
throw new RuntimeException("map does not contain: " + key);
return map.remove(key);
}
public String toString() {
return toString(defaultEquals, defaultEndOfLine);
}
// maybe we don;t need this stuff
// we have tests for it though
// it might be useful
public String toString(final String equals, final String endOfLine) {
final StringBuffer sb = new StringBuffer();
for (Map.Entry<T, Object> entry : map.entrySet())
sb.append(entry.getKey()).append(equals).append(entry.getValue()).append(endOfLine);
return sb.toString();
}
public Properties toProperties() {
final Properties p = new Properties();
for (Map.Entry<T, Object> entry : map.entrySet())
p.put(entry.getKey().toString(), entry.getValue().toString());
return p;
}
public void addStringProperties(final Properties properties) {
// keep this for strings, but mostly do work in the enum class
// i.e. static GenericAttributes<PA> fromProperties();
// which would use a fromString()
for (Map.Entry<Object, Object> entry : properties.entrySet()) {
final String key = (String) entry.getKey();
final String value = (String) entry.getValue();
addProperty(key, value);
}
}
public void addProperty(final String key, final Object value) {
try {
final T e = Enum.valueOf(keyType, key);
map.put(e, value);
} catch (IllegalArgumentException e) {
System.err.println(key + " is not an enum from: " + keyType);
}
}
public int size() {
return map.size();
}
public static Properties load(final InputStream inputStream,final Properties defaultProperties) {
final Properties p=defaultProperties!=null?new Properties(defaultProperties):new Properties();
try {
p.load(inputStream);
} catch(IOException e) {
throw new RuntimeException(e);
}
return p;
}
public static Properties load(final File file,final Properties defaultProperties) {
Properties p=null;
try {
final InputStream is=new FileInputStream(file);
p=load(is,defaultProperties);
is.close();
} catch(IOException e) {
throw new RuntimeException(e);
}
return p;
}
public static void store(final OutputStream outputStream, final Properties properties) {
try {
properties.store(outputStream, null);
} catch (IOException e) {
throw new RuntimeException(e);
}
}
public static void store(final File file, final Properties properties) {
try {
final OutputStream os = new FileOutputStream(file);
store(os, properties);
os.close();
} catch (IOException e) {
throw new RuntimeException(e);
}
}
final Class<T> keyType;
static final String defaultEquals = "=", defaultEndOfLine = "\n";
private final EnumMap<T, Object> map;
public static void main(String[] args) {
}
}
package p;
import static org.junit.Assert.*;
import org.junit.*;
import java.io.*;
import java.util.*;
enum A1 {
foo,bar,baz;
}
enum A2 {
x,y,z;
}
public class GenericAttributesTestCase {
#Test public void testGenericAttributes() {
new GenericAttributes<A1>(A1.class);
}
#Test public void testGenericAttributesKeyTypeProperties() {
final Properties expected=gA1.toProperties();
final GenericAttributes<A1> gA=new GenericAttributes<A1>(A1.class,expected);
final Properties actual=gA.toProperties();
assertEquals(expected,actual);
}
#Test public void testGet() {
final A1 key=A1.foo;
emptyGA1.put(key,null);
final Object actual=emptyGA1.get(key);
assertEquals(null,actual);
}
#Test public void testGetInteger() {
// attributes.add(key,integer);
// assertEquals(integer,attributes.get("key"));
}
#Test public void testContains() {
for(A1 a:A1.values())
assertFalse(emptyGA1.contains(a));
}
#Test public void testChange() {
final A1 key=A1.foo;
final Integer value=42;
emptyGA1.put(key,value);
final Integer expected=43;
emptyGA1.change(key,expected);
final Object actual=emptyGA1.get(key);
assertEquals(expected,actual);
}
#Test public void testAdd() {
final A1 key=A1.foo;
final Integer expected=42;
emptyGA1.put(key,expected);
final Object actual=emptyGA1.get(key);
assertEquals(expected,actual);
}
#Test public void testRemove() {
final A1 key=A1.foo;
final Integer value=42;
emptyGA1.put(key,value);
emptyGA1.remove(key);
assertFalse(emptyGA1.contains(key));
}
#Test public void testToString() {
final String actual=gA1.toString();
final String expected="foo=a foo value\nbar=a bar value\n";
assertEquals(expected,actual);
}
#Test public void testToStringEqualsEndOfLine() {
final String equals=",";
final String endOFLine=";";
final String actual=gA1.toString(equals,endOFLine);
final String expected="foo,a foo value;bar,a bar value;";
assertEquals(expected,actual);
}
#Test public void testEmbedded() {
final String equals=",";
final String endOfLine=";";
//System.out.println("toString(\""+equals+"\",\""+endOFLine+"\"):");
final String embedded=gA1.toString(equals,endOfLine);
GenericAttributes<A2> gA2=new GenericAttributes<A2>(A2.class);
gA2.put(A2.x,embedded);
//System.out.println("embedded:\n"+gA2);
// maybe do file={name=a.jpg;dx=1;zoom=.5}??
// no good, key must be used more than once
// so file:a.jpg={} and hack
// maybe file={name=...} will work
// since we have to treat it specially anyway?
// maybe this is better done in ss first
// to see how it grows?
}
#Test public void testFromString() {
// final Attributes a=Attributes.fromString("");
// final String expected="";
// assertEquals(expected,a.toString());
}
#Test public void testToProperties() {
final Properties expected=new Properties();
expected.setProperty("foo","a foo value");
expected.setProperty("bar","a bar value");
final Properties actual=gA1.toProperties();
assertEquals(expected,actual);
}
#Test public void testAddProperties() {
final Properties p=gA1.toProperties();
final GenericAttributes<A1> ga=new GenericAttributes<A1>(A1.class);
ga.addStringProperties(p);
// assertEquals(ga1,ga); // fails since we need to define equals!
// hack, go backwards
final Properties p2=ga.toProperties();
assertEquals(p,p2); // hack until we define equals
}
#Test public void testStore() throws Exception {
final Properties expected=gA1.toProperties();
final ByteArrayOutputStream baos=new ByteArrayOutputStream();
GenericAttributes.store(baos,expected);
baos.close();
final byte[] bytes=baos.toByteArray();
final ByteArrayInputStream bais=new ByteArrayInputStream(bytes);
final Properties actual=GenericAttributes.load(bais,null);
bais.close();
assertEquals(expected,actual);
}
#Test public void testLoad() throws Exception {
final Properties expected=gA1.toProperties();
final ByteArrayOutputStream baos=new ByteArrayOutputStream();
GenericAttributes.store(baos,expected);
baos.close();
final ByteArrayInputStream bais=new ByteArrayInputStream(baos.toByteArray());
final Properties actual=GenericAttributes.load(bais,null);
bais.close();
assertEquals(expected,actual);
}
#Test public void testMain() {
// fail("Not yet implemented");
}
GenericAttributes<A1> gA1=new GenericAttributes<A1>(A1.class);
{
gA1.put(A1.foo,"a foo value");
gA1.put(A1.bar,"a bar value");
}
GenericAttributes<A1> emptyGA1=new GenericAttributes<A1>(A1.class);
}
answering your comment:
seems like i am getting values by using the enum as the key. i am probably confused.
an enum can implement an interface and each set of enums could have an instance of that base class and delegate calls to it (see item 34 of http://java.sun.com/docs/books/effective/toc.html)
i found the other code that went with my generic attributes (please see below), but i can't find any tests for it and am not quite sure what i was doing other than perhaps to add some stronger typing.
my motivation for all of this was to store some attributes for a photo viewer like picasa, i wanted to store a bunch of attributes for a picture in a single line of a property file
package p;
import java.util.*;
public enum GA {
// like properties, seems like this wants to be constructed with a set of default values
i(Integer.class) {
Integer fromString(final String s) {
return new Integer(s);
}
Integer fromNull() {
return zero; // return empty string?
}
},
b(Boolean.class) {
Boolean fromString(final String s) {
return s.startsWith("t")?true:false;
}
Boolean fromNull() {
return false;
}
},
d(Double.class) {
Double fromString(final String s) {
return new Double(s);
}
Double fromNull() {
return new Double(zero);
}
};
GA() {
this(String.class);
}
GA(final Class clazz) {
this.clazz=clazz;
}
abstract Object fromString(String string);
abstract Object fromNull();
static GenericAttributes<GA> fromProperties(final Properties properties) {
final GenericAttributes<GA> pas=new GenericAttributes<GA>(GA.class);
for(Map.Entry<Object,Object> entry:properties.entrySet()) {
final String key=(String)entry.getKey();
final GA pa=valueOf(key);
if(pa!=null) {
final String stringValue=(String)entry.getValue();
Object value=pa.fromString(stringValue);
pas.addProperty(key,value);
} else throw new RuntimeException(key+"is not a member of "+"GA");
}
return pas;
}
// private final Object defaultValue; // lose type?; require cast?
/* private */final Class clazz;
static final Integer zero=new Integer(0);
}
If you are still looking for answers, you could give a try to Properties library which is open-source with MIT license. Using this, you won't have to specify string constants and everything will be determined by an enum defined by you. And, it has some other features too. Highlights of this library are:
All property keys are defined in a single place, i.e. a user defined enum
Property values can contain variables (starting with $ sign, e.g. $PATH) where PATH is a property key in same file
Property value can be obtained as specified data type, so no need to convert string value to required data type
Property value can be obtained as list of specified data types
Property value can be a multi-line text
Can make property keys mandatory or optional
Can specify default value for the property key if value is not available
Is thread safe
You can find sample programs here