public class TestAssert {
    public static void main(String[] args) {
        int x = 10;
        System.out.println("Testing Assertion that x==100");
        assert x == 100 : "Out assertion failed!";
        System.out.println("Test passed!");
    }
}

// 报错信息如下
Testing Assertion that x==100
Exception in thread "main" java.lang.AssertionError: Out assertion failed!
	at cn.icanci.day0808.TestAssert.main(TestAssert.java:14)

public class TestAssert {
    public static void main(String[] args) {
        Preferences root = Preferences.userRoot();
        root.putInt("age", 10);
        // 这里的1是默认值，当没有获得age的值会返回它
        int fontSize = root.getInt("age", 1);
        System.out.println(fontSize);
    }
}

public class ExceptionHandling {
    public static void main(String[] args) {
        try {
            //创建一个错误
            NumberFormatException ex =
                    new NumberFormatException("Exception");
            //设置错误的触发原因
            ex.initCause(new NullPointerException(
                    "This is actual cause of the exception"));
            //抛出错误并指明原因
            throw ex;
        } catch (NumberFormatException ex) {
            //在控制台打印错误
            System.out.println(ex);
            //获得错误的触发原因
            System.out.println(ex.getCause());
        }
    }
}