void testData() {
 
    //Number包含了int和double
    int a = 4;
    int b = 8;
    print(a + b);
    int a1;
    if (a == null) {
      print('a == null');
    } else {
      print('a != null');
    }
 
    if (a1 == null) {
      print('a1 == null');
    } else {
      print('a1 != null');
    }
 
    double c = 5.9;
    double d = 6.4;
    print(c + d);
 
    //String类型
    var chen = 'chen';
    var yu = 'yu';
    var name = chen + yu;
    print(name);
 
    var hello = '''
    hello word
    public static void main1
    ''';
    print(hello);
 
    var word = """
    hello word
    public stati void main2
    """;
    print(word);
 
 
    //Boolean类型
    bool isSelect = false;
    if (isSelect) {
      print('isSelect is true');
    } else {
      print('isSelect is false');
    }
 
 
    //List类型
    var list = [];
    list.add(1);
    list.add(2);
    print(list);
    print('size is ${list.length}');
 
    list.removeAt(0);
    print(list);
    print('size is ${list.length}');
 
 
    //Map类型
    var week = {'one':'test1', 'two':'test2'};
    print(week);
    print('week length is ${week.length}');
    week.putIfAbsent('three', () => 'test3');
    print(week);
    print('week length is ${week.length}');
  }

I/flutter (24359): 12
I/flutter (24359): a != null
I/flutter (24359): a1 == null
I/flutter (24359): 12.3
I/flutter (24359): chenyu
I/flutter (24359):     hello word
I/flutter (24359):     public static void main1
I/flutter (24359):     
I/flutter (24359):     hello word
I/flutter (24359):     public stati void main2
I/flutter (24359):     
I/flutter (24359): isSelect is false
I/flutter (24359): [1, 2]
I/flutter (24359): size is 2
I/flutter (24359): [2]
I/flutter (24359): size is 1
I/flutter (24359): {one: test1, two: test2}
I/flutter (24359): week length is 2
I/flutter (24359): {one: test1, two: test2, three: test3}
I/flutter (24359): week length is 3