java 各种数据类型判断为空

// 判断String为空
//String s = "";
//String s = null;
String s = " ";
s = s.trim(); // 处理" "
// 方法1
if (s == null || "".equals(s)) { // null,""," "
System.out.println("String 为空1");
}
// 方法2
if (s == null || s.length() <= 0) { // 推荐 // null,""," "
System.out.println("String 为空2");
} else if (s.length() == 1 && Character.isWhitespace(s.charAt(0)) == true) {
System.out.println("String 为空5");
}
// 方法3
if (s == null || s.isEmpty()) { // null,""," "
System.out.println("String 为空3");
}
// 方法4
if (s == null || s == "") { // 不推荐 // null,""
System.out.println("String 为空4");
}

// 判断数组为空
//String[] arr = null;
String[] arr = new String[0];
//String[] arr = new String[2]; // 有长度,不为空
if (arr == null || arr.length == 0) {
System.out.println("数组 为空1");
}

// 判断list为空（Map、Set同list）
//List<String> strList = null;
List<String> strList = new ArrayList<String>();
if (strList == null || strList.size() == 0) {
System.out.println("list 为空1");
}
if (strList == null || strList.isEmpty()) { // 推荐
System.out.println("list 为空2");
}