Guava源码阅读计划:base.Strings.java

com.google.common.base：基本工具类库和接口。 

import com.google.common.annotations.VisibleForTesting;

public final class Strings {
    private Strings() {
    }

    public static boolean isNullOrEmpty(@Nullable String string) {
        // string.isEmpty() in Java 6 
        // 判断 == 0 的情况时 应先判断是否为空
        return string == null || string.length() == 0;
    }

    //重复添加单个元素到字符串开头,直到该字符串长度为minLength中，通过StringBuilder.append实现的
    public static String padStart(String string, int minLength, char padChar) {
        checkNotNull(string);  // eager for GWT.
        if (string.length() >= minLength) {
            return string;
        }
        StringBuilder sb = new StringBuilder(minLength);
        for (int i = string.length(); i < minLength; i++) {
            sb.append(padChar);
        }
        sb.append(string);
        return sb.toString();
    }

    //与上个方法类似，重复添加到结尾
    public static String padEnd(String string, int minLength, char padChar) {
        checkNotNull(string);  // eager for GWT.
        if (string.length() >= minLength) {
            return string;
        }
        StringBuilder sb = new StringBuilder(minLength);
        sb.append(string);
        for (int i = string.length(); i < minLength; i++) {
            sb.append(padChar);
        }
        return sb.toString();
    }


    //将String重复count次
    public static String repeat(String string, int count) {
        checkNotNull(string);  // eager for GWT.

        if (count <= 1) {
            checkArgument(count >= 0, "invalid count: %s", count);
            return (count == 0) ? "" : string;
        }

        // IF YOU MODIFY THE CODE HERE, you must update StringsRepeatBenchmark
        final int len = string.length();
        final long longSize = (long) len * (long) count;
        final int size = (int) longSize;
        if (size != longSize) {
            throw new ArrayIndexOutOfBoundsException(
                    "Required array size too large: " + longSize);
        }

        final char[] array = new char[size];
        string.getChars(0, len, array, 0);
        int n;
        //巧妙的算法设计 每次尽可能多的添加元素
        for (n = len; n < size - n; n <<= 1) {
            System.arraycopy(array, 0, array, n, n);
        }
        //当不足以在添加N个元素时跳出循环,再添加(size - n)个元素
        System.arraycopy(array, 0, array, n, size - n);
        return new String(array);
    }

    //repeat中使用的arraycopy方法上有关键词native,
    //表示这个方法是使用汇编或者其他底层语言实现过的。所以肯定要比我们自己写的Java代码效率上要高
    public static native void arraycopy(Object src, int srcPos, Object dest, int destPos, int length);

    //寻找公共前缀
    public static String commonPrefix(CharSequence a, CharSequence b) {
        checkNotNull(a);
        checkNotNull(b);

        int maxPrefixLength = Math.min(a.length(), b.length());
        int p = 0;
        while (p < maxPrefixLength && a.charAt(p) == b.charAt(p)) {
            p++;
        }
        if (validSurrogatePairAt(a, p - 1) || validSurrogatePairAt(b, p - 1)) {
            p--;
        }
        return a.subSequence(0, p).toString();
    }


    //寻找公共后缀
    public static String commonSuffix(CharSequence a, CharSequence b) {
        checkNotNull(a);
        checkNotNull(b);

        int maxSuffixLength = Math.min(a.length(), b.length());
        int s = 0;
        while (s < maxSuffixLength
                && a.charAt(a.length() - s - 1) == b.charAt(b.length() - s - 1)) {
            s++;
        }
        if (validSurrogatePairAt(a, a.length() - s - 1)
                || validSurrogatePairAt(b, b.length() - s - 1)) {
            s--;
        }
        return a.subSequence(a.length() - s, a.length()).toString();
    }


    @VisibleForTesting
    static boolean validSurrogatePairAt(CharSequence string, int index) {
        return index >= 0 && index <= (string.length() - 2)
                && Character.isHighSurrogate(string.charAt(index))
                && Character.isLowSurrogate(string.charAt(index + 1));
    }
}