java：==、equals、equalsIgnoreCase的作用以及区别

1. ==

“==”是关系运算符，结果返回布尔值

“==”使用情况如下：

​     a)基本类型，比较的是值

​     b)引用类型，比较的是地址

​     c)不能比较没有父子关系的两个对象

2. equals

equals()是方法，结果返回布尔值

object方法的源码：

 public boolean equals(Object obj) {
    
    
        return (this == obj);
    }

string类重写了这个方法：

public boolean equals(Object anObject) {
    
    
        if (this == anObject) {
    
    
            return true;
        }
        if (anObject instanceof String) {
    
    
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
    
    
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
    
    
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

总结：
    a) equals方法默认比较的是地址
    b) string重写以后，首先还是比较地址，如果地址相同那么直接返回true，如果地址不同，比较值是否相同。

3. equalsIgnoreCase

public boolean equalsIgnoreCase(String anotherString) {
    
    
    return (this == anotherString) ? true
            : (anotherString != null)
            && (anotherString.value.length == value.length)
            && regionMatches(true, 0, anotherString, 0, value.length);
}
public boolean regionMatches(boolean ignoreCase, int toffset,
        String other, int ooffset, int len) {
    
    
    char ta[] = value;
    int to = toffset;
    char pa[] = other.value;
    int po = ooffset;
    // Note: toffset, ooffset, or len might be near -1>>>1.
    if ((ooffset < 0) || (toffset < 0)
            || (toffset > (long)value.length - len)
            || (ooffset > (long)other.value.length - len)) {
    
    
        return false;
    }
    while (len-- > 0) {
    
    
        char c1 = ta[to++];
        char c2 = pa[po++];
        if (c1 == c2) {
    
    
            continue;
        }
        if (ignoreCase) {
    
    
            // If characters don't match but case may be ignored,
            // try converting both characters to uppercase.
            // If the results match, then the comparison scan should
            // continue.
            char u1 = Character.toUpperCase(c1);
            char u2 = Character.toUpperCase(c2);
            if (u1 == u2) {
    
    
                continue;
            }
            // Unfortunately, conversion to uppercase does not work properly
            // for the Georgian alphabet, which has strange rules about case
            // conversion.  So we need to make one last check before
            // exiting.
            if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
    
    
                continue;
            }
        }
        return false;
    }
    return true;
}

代码思路比较清晰， 比较步骤分三步：

​     a) 使用==比较地址，

​     b) 字符串全部转为大写，比较值

​     c) 字符串全部转为小写，比较值

这里先转为大写比较，又转为小写比较，是因为注释中的一段话：

    //nfortunately, conversion to uppercase does not work properly
    // for the Georgian alphabet, which has strange rules about case
    // conversion.  So we need to make one last check before
    // exiting.