CoreJavaE10V1P3.6 第3章 Java的基本编程结构－3.6 字符串 String

String greeting = "Hello";
String s = greeting.substring(0, 3);
s结果为“Hel”

int age = 13;
String rating = "PG" + age;
rating 结果为 "PG13"

greeting = greeting.substring(0, 3) + "p!";

s.equals(t)
"Hello".equals(greeting)
"Hello".equalsIgnoreCase("hello")
equals 用于判断两个字符串的值是否相等String greeting = "Hello"; //initialize greeting to a string
if (greeting == "Hello") . . .
// probably true
if (greeting.substring(0, 3) == "Hel") . . .
// probably false
== 用于判断两个是否为同一个字符串，即存储地址是否相同

if (str.length() == 0)
if (str.equals(""))

if (str != null && str.length() != 0)

String greeting = "Hello";
int n = greeting.length(); // is 5.

int cpCount = greeting.codePointCount(0, greeting.length());

char first = greeting.charAt(0); // first is 'H'
char last = greeting.charAt(4); // last is 'o'

int index = greeting.offsetByCodePoints(0, i);
int cp = greeting.codePointAt(index);

char charAt(int index)
returns the code unit at the specified location.You probably don’t want to call this method unless you are interested in low-level code units.
• int codePointAt(int index) 5.0
returns the code point that starts at the specified location.
• int offsetByCodePoints(int startIndex, int cpCount) 5.0
returns the index of the code point that is cpCount code points away from the code point at startIndex .
• int compareTo(String other)
returns a negative value if the string comes before other in dictionary order, a positive value if the string comes after other in dictionary order, or 0 if the strings are equal.
• IntStream codePoints() 8
returns the code points of this string as a stream. Call toArray to put them in an array.
• new String(int[] codePoints, int offset, int count) 5.0
constructs a string with the count code points in the array starting at offset .
• boolean equals(Object other)
returns true if the string equals other .
• boolean equalsIgnoreCase(String other)
returns true if the string equals other , except for upper/lowercase distinction.
• boolean startsWith(String prefix)
• boolean endsWith(String suffix)
returns true if the string starts or ends with suffix int indexOf(String str)
• int indexOf(String str, int fromIndex)
• int indexOf(int cp)
• int indexOf(int cp, int fromIndex)
returns the start of the first substring equal to the string str or the code point cp ,starting at index 0 or at fromIndex , or -1 if str does not occur in this string.
• int lastIndexOf(String str)
• int lastIndexOf(String str, int fromIndex)
• int lastindexOf(int cp)
• int lastindexOf(int cp, int fromIndex)
returns the start of the last substring equal to the string str or the code point cp ,starting at the end of the string or at fromIndex .
• int length()
returns the number of code units of the string.
• int codePointCount(int startIndex, int endIndex) 5.0
returns the number of code points between startIndex and endIndex - 1 .
• String replace(CharSequence oldString, CharSequence newString)
returns a new string that is obtained by replacing all substrings matching oldString in the string with the string newString .You can supply String or StringBuilder objects
for the CharSequence parameters.
• String substring(int beginIndex)
• String substring(int beginIndex, int endIndex)
returns a new string consisting of all code units from beginIndex until the end of the string or until endIndex - 1 .
• String toLowerCase()
String toUpperCase()
returns a new string containing all characters in the original string, with uppercase characters converted to lowercase, or lowercase characters converted to uppercase.
• String trim()
returns a new string by eliminating all leading and trailing whitespace in the original string.
• String join(CharSequence delimiter, CharSequence... elements) 8 Returns a new string joining all elements with the given delimiter.

StringBuilder builder = new StringBuilder();
builder.append(ch); // appends a single character
builder.append(str); // appends a string
String completedString = builder.toString();