FormAPI 2.1-SNAPSHOT

    SimpleForm form = new SimpleForm("Test form")
    .setContent("Some kind of test text should be written here.")
    .addButton("Button")
    .addButton("Button with a picture", ImageType.PATH, "textures/items/diamond");

form.send(player, (targetPlayer, targetForm, data) -> {
    if(data == -1) return; //If the form is closed forcibly, then data will have a value of -1

    targetPlayer.sendMessage(data.toString());
});

CustomForm form = new CustomForm()
    .addLabel("Here some random text should be written, but it is written.")
    .addDropDown("Drop-down list", Arrays.asList("Var 1", "Var 2", "Var 3"))
    .addInput("Text field")
    .addSlider("Slider", 1, 100)
    .addStepSlider("Step slider", Arrays.asList("Var 1", "Var 2", "Var 3"))
    .addToggle("Toggle", false);

form.send(player, (targetPlayer, targetForm, data) -> {
    if(data == null) return; //If the form is closed forcibly, then data will have a value of null

    targetPlayer.sendMessage(data.toString());
});

ModalForm form = new ModalForm("HEADER", "TEXT", "BUTTON 1", "BUTTON 2");

form.send(player, (targetPlayer, targetForm, data) -> {
    if(data == -1) return; //If the form is closed forcibly, then data will have a value of -1

    targetPlayer.sendMessage(data.toString());
});