class AddMethodCommand implements Runnable {

    private final PsiClass psiClass;
    private final String methodSignatureAndBody;

    public AddMethodCommand(PsiClass psiClass, String methodSignatureAndBody) {
        this.psiClass = psiClass;
        this.methodSignatureAndBody = methodSignatureAndBody;
    }

    public void run() {
        ApplicationManager.getApplication().runWriteAction(new Runnable() {
            public void run() {
                try {
                    PsiManager psiManager = getPsiManager();
                    PsiMethod newMethod = psiManager.getElementFactory().createMethodFromText(methodSignatureAndBody, psiClass);
                    PsiMethod methodAnchor = findNearestMethodAnchorInSelectedFile();
                    if (methodAnchor != null) {
                        methodAnchor.getParent().addAfter(newMethod, methodAnchor);
                    } else {
                        psiClass.getNavigationElement().add(newMethod);
                    }

                    psiManager.getCodeStyleManager().reformat(newMethod);
                    updateDocument(psiClass.getContainingFile());
                    newMethod = psiClass.findMethodBySignature(newMethod, false);
                    jumpToPsiElement(newMethod);

                    PsiWhiteSpace whiteSpace = (PsiWhiteSpace) PsiTreeUtil.getChildOfType(newMethod.getBody(), PsiWhiteSpace.class);
                    TextRange range = whiteSpace.getTextRange();
                    getSelectedTextEditor().getCaretModel().moveToOffset(range.getStartOffset());
                    getSelectedTextEditor().getSelectionModel().setSelection(range.getStartOffset(), range.getEndOffset());
                } catch (IncorrectOperationException e) {
                    LOGGER.error(e);
                    showErrorMessage("Could not add method: " + e.getMessage(), "Warning");
                }
            }
        });
    }

    private PsiMethod findNearestMethodAnchorInSelectedFile() {
        PsiFile psiFile = getPsiDocumentManager().getPsiFile(getSelectedTextEditor().getDocument());
        int offset = getSelectedTextEditor().getCaretModel().getOffset();
        PsiElement currentElement = psiFile.findElementAt(offset);
        if (currentElement == null) {
            return null;
        }

        PsiElement nearestMethod = PsiTreeUtil.getParentOfType(currentElement, PsiMethod.class);
        if (nearestMethod == null) {
            nearestMethod = PsiTreeUtil.getPrevSiblingOfType(currentElement, PsiMethod.class);
        }
        if (nearestMethod == null) {
            nearestMethod = PsiTreeUtil.getNextSiblingOfType(currentElement, PsiMethod.class);
        }
        return (PsiMethod) nearestMethod;
    }
}

This command is executed using the following statement:

CommandProcessor.getInstance().executeCommand(project, command, name, commandID);