问如何通过在java中添加char by char从char数组创建字符串

EN

String t = "foobarbaz";
int l = t.length();
char[] a;
a = new char[l];
String rel = ",";
String ret = "";
for (int i = 0; i<l; i++){
    a[i] = t.charAt(i);
}
for (int v = 0; v<l; v++){
    ret += a[v];
    ret += rel;
}
System.out.println(ret);

String s = "abcdefg";

StringBuilder builder = new StringBuilder();

for (char c : s.toCharArray()) {
    builder.append(c).append(",");
}

// Alternatively, you can do it in this way
for (String symbol : s.split("")) {
    builder.append(symbol).append(",");
}

System.out.println(builder.toString());

// Java 8 (the result string doesn't have a comma at the end)
String collect = Arrays.stream(s.split("")).collect(Collectors.joining(","));

// Java8 StringJoiner
StringJoiner sj = new StringJoiner(",");
// StringJoiner sj = new StringJoiner(",", "", ",");
for (String str : s.split("")) {
    sj.add(str);
}

String t;
int l = t.length();
char[] a;
a = new char[l];
String rel = ",";
String ret = null; //you initialize ret to null, it should be "";

for (int i = 0; i<l; i++){
    //you always set it to the character at position 0, you should do t.charAt(i)
    //you don't need to use the wrapper class just t.charAt(i) will be fine.
    a[i] = new Character(t.charAt(0)); 
}

for (int v = 0; v<l; v--){//you decrement v instead of incrementing it, this will lead to exceptions
    ret += a[v];
    ret += rel;//you always add the delimiter, note that this will lead to a trailing delimiter at the end
}

String t = "Test";
StringBuilder builder = new StringBuilder();
if(t.length() > 0){
    builder.append(t.charAt(0));
    for(int i=1;i<t.length();i++){
        builder.append(",");
        builder.append(t.charAt(i));
    }
}
System.out.println(builder.toString());