Example showing how to generate, compile, load and run Java programs at run-time.

CodeGenMeta.java

import javax.tools.*;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.lang.reflect.Method;
import java.net.URI;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

public class CodeGenMeta {
  public static void main(String[] args) throws Exception {
    String program = "" +
        "public class CodeGenTest {\n" +
        "  public static void main(String[] args) {\n" +
        "    System.out.println(\"Hello World, from a generated program!\");\n" +
        "  }\n" +
        "}\n";

    JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

    JavaFileObject compilationUnit =
        new StringJavaFileObject("CodeGenTest", program);

    SimpleJavaFileManager fileManager =
        new SimpleJavaFileManager(compiler.getStandardFileManager(null, null, null));

    JavaCompiler.CompilationTask compilationTask = compiler.getTask(
        null, fileManager, null, null, null, Arrays.asList(compilationUnit));

    compilationTask.call();

    CompiledClassLoader classLoader =
        new CompiledClassLoader(fileManager.getGeneratedOutputFiles());

    Class<?> codeGenTest = classLoader.loadClass("CodeGenTest");
    Method main = codeGenTest.getMethod("main", String[].class);
    main.invoke(null, new Object[]{null});
  }

  private static class StringJavaFileObject extends SimpleJavaFileObject {
    private final String code;

    public StringJavaFileObject(String name, String code) {
      super(URI.create("string:///" + name.replace('.', '/') + Kind.SOURCE.extension),
          Kind.SOURCE);
      this.code = code;
    }

    @Override
    public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOException {
      return code;
    }
  }

  private static class ClassJavaFileObject extends SimpleJavaFileObject {
    private final ByteArrayOutputStream outputStream;
    private final String className;

    protected ClassJavaFileObject(String className, Kind kind) {
      super(URI.create("mem:///" + className.replace('.', '/') + kind.extension), kind);
      this.className = className;
      outputStream = new ByteArrayOutputStream();
    }

    @Override
    public OutputStream openOutputStream() throws IOException {
      return outputStream;
    }

    public byte[] getBytes() {
      return outputStream.toByteArray();
    }

    public String getClassName() {
      return className;
    }
  }

  private static class SimpleJavaFileManager extends ForwardingJavaFileManager {
    private final List<ClassJavaFileObject> outputFiles;

    protected SimpleJavaFileManager(JavaFileManager fileManager) {
      super(fileManager);
      outputFiles = new ArrayList<ClassJavaFileObject>();
    }

    @Override
    public JavaFileObject getJavaFileForOutput(Location location, String className, JavaFileObject.Kind kind, FileObject sibling) throws IOException {
      ClassJavaFileObject file = new ClassJavaFileObject(className, kind);
      outputFiles.add(file);
      return file;
    }

    public List<ClassJavaFileObject> getGeneratedOutputFiles() {
      return outputFiles;
    }
  }

  private static class CompiledClassLoader extends ClassLoader {
    private final List<ClassJavaFileObject> files;

    private CompiledClassLoader(List<ClassJavaFileObject> files) {
      this.files = files;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
      Iterator<ClassJavaFileObject> itr = files.iterator();
      while (itr.hasNext()) {
        ClassJavaFileObject file = itr.next();
        if (file.getClassName().equals(name)) {
          itr.remove();
          byte[] bytes = file.getBytes();
          return super.defineClass(name, bytes, 0, bytes.length);
        }
      }
      return super.findClass(name);
    }
  }
}

returning Exception in thread "main" java.lang.NullPointerException
at net.photon.smart.CodeGenMeta.main(CodeGenMeta.java:28)

Exception in thread "main" java.lang.NullPointerException
at com.mvit.advancedjava.CollectionGenericsTypeExctractor.main(CollectionGenericsTypeExctractor.java:29)

what if the .java file consists of import statements.
when I tried it showed no such class found?